diff --git a/core/field_dropdown.js b/core/field_dropdown.js
index 5dad752d8..ddb5a4e18 100644
--- a/core/field_dropdown.js
+++ b/core/field_dropdown.js
@@ -12,21 +12,22 @@
  */
 'use strict';
 
-goog.provide('Blockly.FieldDropdown');
+goog.module('Blockly.FieldDropdown');
+goog.module.declareLegacyNamespace();
 
-goog.require('Blockly.DropDownDiv');
-goog.require('Blockly.Field');
-goog.require('Blockly.fieldRegistry');
-goog.require('Blockly.Menu');
-goog.require('Blockly.MenuItem');
-goog.require('Blockly.utils');
-goog.require('Blockly.utils.aria');
-goog.require('Blockly.utils.Coordinate');
-goog.require('Blockly.utils.dom');
-goog.require('Blockly.utils.object');
-goog.require('Blockly.utils.string');
-goog.require('Blockly.utils.Svg');
-goog.require('Blockly.utils.userAgent');
+const Coordinate = goog.require('Blockly.utils.Coordinate');
+const DropDownDiv = goog.require('Blockly.DropDownDiv');
+const Field = goog.require('Blockly.Field');
+const Menu = goog.require('Blockly.Menu');
+const MenuItem = goog.require('Blockly.MenuItem');
+const Svg = goog.require('Blockly.utils.Svg');
+const aria = goog.require('Blockly.utils.aria');
+const dom = goog.require('Blockly.utils.dom');
+const fieldRegistry = goog.require('Blockly.fieldRegistry');
+const userAgent = goog.require('Blockly.utils.userAgent');
+const {commonWordPrefix, commonWordSuffix, shortestStringLength} = goog.require('Blockly.utils.string');
+const {inherits} = goog.require('Blockly.utils.object');
+const {replaceMessageReferences} = goog.require('Blockly.utils');
 
 
 /**
@@ -38,22 +39,23 @@ goog.require('Blockly.utils.userAgent');
  *    option & returns a validated language-neutral dropdown option, or null to
  *    abort the change.
  * @param {Object=} opt_config A map of options used to configure the field.
- *    See the [field creation documentation]{@link https://developers.google.com/blockly/guides/create-custom-blocks/fields/built-in-fields/dropdown#creation}
+ *    See the [field creation documentation]{@link
+ *    https://developers.google.com/blockly/guides/create-custom-blocks/fields/built-in-fields/dropdown#creation}
  *    for a list of properties this parameter supports.
- * @extends {Blockly.Field}
+ * @extends {Field}
  * @constructor
  * @throws {TypeError} If `menuGenerator` options are incorrectly structured.
  */
-Blockly.FieldDropdown = function(menuGenerator, opt_validator, opt_config) {
+const FieldDropdown = function(menuGenerator, opt_validator, opt_config) {
   if (typeof menuGenerator != 'function') {
-    Blockly.FieldDropdown.validateOptions_(menuGenerator);
+    validateOptions(menuGenerator);
   }
 
   /**
    * An array of options for a dropdown list,
    * or a function which generates these options.
    * @type {(!Array<!Array>|
-   *    !function(this:Blockly.FieldDropdown): !Array<!Array>)}
+   *    !function(this:FieldDropdown): !Array<!Array>)}
    * @protected
    */
   this.menuGenerator_ = menuGenerator;
@@ -90,19 +92,19 @@ Blockly.FieldDropdown = function(menuGenerator, opt_validator, opt_config) {
   this.selectedOption_ = this.getOptions(false)[0];
 
   // Call parent's constructor.
-  Blockly.FieldDropdown.superClass_.constructor.call(
+  FieldDropdown.superClass_.constructor.call(
       this, this.selectedOption_[1], opt_validator, opt_config);
 
   /**
    * A reference to the currently selected menu item.
-   * @type {?Blockly.MenuItem}
+   * @type {?MenuItem}
    * @private
    */
   this.selectedMenuItem_ = null;
 
   /**
    * The dropdown menu.
-   * @type {?Blockly.Menu}
+   * @type {?Menu}
    * @protected
    */
   this.menu_ = null;
@@ -128,27 +130,27 @@ Blockly.FieldDropdown = function(menuGenerator, opt_validator, opt_config) {
    */
   this.svgArrow_ = null;
 };
-Blockly.utils.object.inherits(Blockly.FieldDropdown, Blockly.Field);
+inherits(FieldDropdown, Field);
 
 /**
  * Dropdown image properties.
  * @typedef {{
-  *            src:string,
-  *            alt:string,
-  *            width:number,
-  *            height:number
-  *          }}
-  */
-Blockly.FieldDropdown.ImageProperties;
+ *            src:string,
+ *            alt:string,
+ *            width:number,
+ *            height:number
+ *          }}
+ */
+FieldDropdown.ImageProperties;
 
 /**
  * Construct a FieldDropdown from a JSON arg object.
  * @param {!Object} options A JSON object with options (options).
- * @return {!Blockly.FieldDropdown} The new field instance.
+ * @return {!FieldDropdown} The new field instance.
  * @package
  * @nocollapse
  */
-Blockly.FieldDropdown.fromJson = function(options) {
+FieldDropdown.fromJson = function(options) {
   // `this` might be a subclass of FieldDropdown if that class doesn't override
   // the static fromJson method.
   return new this(options['options'], undefined, options);
@@ -161,7 +163,7 @@ Blockly.FieldDropdown.fromJson = function(options) {
  *    field's state.
  * @package
  */
-Blockly.FieldDropdown.prototype.fromXml = function(fieldElement) {
+FieldDropdown.prototype.fromXml = function(fieldElement) {
   if (this.isOptionListDynamic()) {
     this.getOptions(false);
   }
@@ -173,52 +175,48 @@ Blockly.FieldDropdown.prototype.fromXml = function(fieldElement) {
  * are not. Editable fields should also be serializable.
  * @type {boolean}
  */
-Blockly.FieldDropdown.prototype.SERIALIZABLE = true;
+FieldDropdown.prototype.SERIALIZABLE = true;
 
 /**
  * Horizontal distance that a checkmark overhangs the dropdown.
  */
-Blockly.FieldDropdown.CHECKMARK_OVERHANG = 25;
+FieldDropdown.CHECKMARK_OVERHANG = 25;
 
 /**
  * Maximum height of the dropdown menu, as a percentage of the viewport height.
  */
-Blockly.FieldDropdown.MAX_MENU_HEIGHT_VH = 0.45;
+FieldDropdown.MAX_MENU_HEIGHT_VH = 0.45;
 
 /**
  * The y offset from the top of the field to the top of the image, if an image
  * is selected.
  * @type {number}
  * @const
- * @private
  */
-Blockly.FieldDropdown.IMAGE_Y_OFFSET = 5;
+const IMAGE_Y_OFFSET = 5;
 
 /**
  * The total vertical padding above and below an image.
  * @type {number}
  * @const
- * @private
  */
-Blockly.FieldDropdown.IMAGE_Y_PADDING =
-    Blockly.FieldDropdown.IMAGE_Y_OFFSET * 2;
+const IMAGE_Y_PADDING = IMAGE_Y_OFFSET * 2;
 
 /**
  * Android can't (in 2014) display "▾", so use "▼" instead.
  */
-Blockly.FieldDropdown.ARROW_CHAR =
-    Blockly.utils.userAgent.ANDROID ? '\u25BC' : '\u25BE';
+FieldDropdown.ARROW_CHAR = userAgent.ANDROID ? '\u25BC' : '\u25BE';
 
 /**
  * Mouse cursor style when over the hotspot that initiates the editor.
  */
-Blockly.FieldDropdown.prototype.CURSOR = 'default';
+FieldDropdown.prototype.CURSOR = 'default';
 
 /**
  * Create the block UI for this dropdown.
  * @package
  */
-Blockly.FieldDropdown.prototype.initView = function() {
+FieldDropdown.prototype.initView = function() {
   if (this.shouldAddBorderRect_()) {
     this.createBorderRect_();
   } else {
@@ -226,8 +224,7 @@ Blockly.FieldDropdown.prototype.initView = function() {
   }
   this.createTextElement_();
 
-  this.imageElement_ = Blockly.utils.dom.createSvgElement(
-      Blockly.utils.Svg.IMAGE, {}, this.fieldGroup_);
+  this.imageElement_ = dom.createSvgElement(Svg.IMAGE, {}, this.fieldGroup_);
 
   if (this.getConstants().FIELD_DROPDOWN_SVG_ARROW) {
     this.createSVGArrow_();
@@ -236,7 +233,7 @@ Blockly.FieldDropdown.prototype.initView = function() {
   }
 
   if (this.borderRect_) {
-    Blockly.utils.dom.addClass(this.borderRect_, 'blocklyDropdownRect');
+    dom.addClass(this.borderRect_, 'blocklyDropdownRect');
   }
 };
 
@@ -245,23 +242,21 @@ Blockly.FieldDropdown.prototype.initView = function() {
  * @return {boolean} True if the dropdown field should add a border rect.
  * @protected
  */
-Blockly.FieldDropdown.prototype.shouldAddBorderRect_ = function() {
+FieldDropdown.prototype.shouldAddBorderRect_ = function() {
   return !this.getConstants().FIELD_DROPDOWN_NO_BORDER_RECT_SHADOW ||
       (this.getConstants().FIELD_DROPDOWN_NO_BORDER_RECT_SHADOW &&
-          !this.sourceBlock_.isShadow());
+       !this.sourceBlock_.isShadow());
 };
 
 /**
  * Create a tspan based arrow.
  * @protected
  */
-Blockly.FieldDropdown.prototype.createTextArrow_ = function() {
-  this.arrow_ = Blockly.utils.dom.createSvgElement(
-      Blockly.utils.Svg.TSPAN, {}, this.textElement_);
+FieldDropdown.prototype.createTextArrow_ = function() {
+  this.arrow_ = dom.createSvgElement(Svg.TSPAN, {}, this.textElement_);
   this.arrow_.appendChild(document.createTextNode(
-      this.sourceBlock_.RTL ?
-      Blockly.FieldDropdown.ARROW_CHAR + ' ' :
-      ' ' + Blockly.FieldDropdown.ARROW_CHAR));
+      this.sourceBlock_.RTL ? FieldDropdown.ARROW_CHAR + ' ' :
+                              ' ' + FieldDropdown.ARROW_CHAR));
   if (this.sourceBlock_.RTL) {
     this.textElement_.insertBefore(this.arrow_, this.textContent_);
   } else {
@@ -273,13 +268,15 @@ Blockly.FieldDropdown.prototype.createTextArrow_ = function() {
  * Create an SVG based arrow.
  * @protected
  */
-Blockly.FieldDropdown.prototype.createSVGArrow_ = function() {
-  this.svgArrow_ = Blockly.utils.dom.createSvgElement(
-      Blockly.utils.Svg.IMAGE, {
+FieldDropdown.prototype.createSVGArrow_ = function() {
+  this.svgArrow_ = dom.createSvgElement(
+      Svg.IMAGE, {
         'height': this.getConstants().FIELD_DROPDOWN_SVG_ARROW_SIZE + 'px',
         'width': this.getConstants().FIELD_DROPDOWN_SVG_ARROW_SIZE + 'px'
-      }, this.fieldGroup_);
-  this.svgArrow_.setAttributeNS(Blockly.utils.dom.XLINK_NS, 'xlink:href',
+      },
+      this.fieldGroup_);
+  this.svgArrow_.setAttributeNS(
+      dom.XLINK_NS, 'xlink:href',
       this.getConstants().FIELD_DROPDOWN_SVG_ARROW_DATAURI);
 };
 
@@ -289,31 +286,29 @@ Blockly.FieldDropdown.prototype.createSVGArrow_ = function() {
  *     or undefined if triggered programmatically.
  * @protected
  */
-Blockly.FieldDropdown.prototype.showEditor_ = function(opt_e) {
+FieldDropdown.prototype.showEditor_ = function(opt_e) {
   this.dropdownCreate_();
   if (opt_e && typeof opt_e.clientX === 'number') {
-    this.menu_.openingCoords =
-        new Blockly.utils.Coordinate(opt_e.clientX, opt_e.clientY);
+    this.menu_.openingCoords = new Coordinate(opt_e.clientX, opt_e.clientY);
   } else {
     this.menu_.openingCoords = null;
   }
   // Element gets created in render.
-  this.menu_.render(Blockly.DropDownDiv.getContentDiv());
-  var menuElement = /** @type {!Element} */ (this.menu_.getElement());
-  Blockly.utils.dom.addClass(menuElement, 'blocklyDropdownMenu');
+  this.menu_.render(DropDownDiv.getContentDiv());
+  const menuElement = /** @type {!Element} */ (this.menu_.getElement());
+  dom.addClass(menuElement, 'blocklyDropdownMenu');
 
   if (this.getConstants().FIELD_DROPDOWN_COLOURED_DIV) {
-    var primaryColour = (this.sourceBlock_.isShadow()) ?
+    const primaryColour = (this.sourceBlock_.isShadow()) ?
         this.sourceBlock_.getParent().getColour() :
         this.sourceBlock_.getColour();
-    var borderColour = (this.sourceBlock_.isShadow()) ?
+    const borderColour = (this.sourceBlock_.isShadow()) ?
         this.sourceBlock_.getParent().style.colourTertiary :
         this.sourceBlock_.style.colourTertiary;
-    Blockly.DropDownDiv.setColour(primaryColour, borderColour);
+    DropDownDiv.setColour(primaryColour, borderColour);
   }
 
-  Blockly.DropDownDiv.showPositionedByField(
-      this, this.dropdownDispose_.bind(this));
+  DropDownDiv.showPositionedByField(this, this.dropdownDispose_.bind(this));
 
   // Focusing needs to be handled after the menu is rendered and positioned.
   // Otherwise it will cause a page scroll to get the misplaced menu in
@@ -331,25 +326,25 @@ Blockly.FieldDropdown.prototype.showEditor_ = function(opt_e) {
  * Create the dropdown editor.
  * @private
  */
-Blockly.FieldDropdown.prototype.dropdownCreate_ = function() {
-  var menu = new Blockly.Menu();
-  menu.setRole(Blockly.utils.aria.Role.LISTBOX);
+FieldDropdown.prototype.dropdownCreate_ = function() {
+  const menu = new Menu();
+  menu.setRole(aria.Role.LISTBOX);
   this.menu_ = menu;
 
-  var options = this.getOptions(false);
+  const options = this.getOptions(false);
   this.selectedMenuItem_ = null;
-  for (var i = 0; i < options.length; i++) {
-    var content = options[i][0];  // Human-readable text or image.
-    var value = options[i][1];    // Language-neutral value.
+  for (let i = 0; i < options.length; i++) {
+    let content = options[i][0];  // Human-readable text or image.
+    const value = options[i][1];  // Language-neutral value.
     if (typeof content == 'object') {
       // An image, not text.
-      var image = new Image(content['width'], content['height']);
+      const image = new Image(content['width'], content['height']);
       image.src = content['src'];
       image.alt = content['alt'] || '';
       content = image;
     }
-    var menuItem = new Blockly.MenuItem(content, value);
-    menuItem.setRole(Blockly.utils.aria.Role.OPTION);
+    const menuItem = new MenuItem(content, value);
+    menuItem.setRole(aria.Role.OPTION);
     menuItem.setRightToLeft(this.sourceBlock_.RTL);
     menuItem.setCheckable(true);
     menu.addChild(menuItem);
@@ -365,7 +360,7 @@ Blockly.FieldDropdown.prototype.dropdownCreate_ = function() {
  * Disposes of events and DOM-references belonging to the dropdown editor.
  * @private
  */
-Blockly.FieldDropdown.prototype.dropdownDispose_ = function() {
+FieldDropdown.prototype.dropdownDispose_ = function() {
   if (this.menu_) {
     this.menu_.dispose();
   }
@@ -376,21 +371,21 @@ Blockly.FieldDropdown.prototype.dropdownDispose_ = function() {
 
 /**
  * Handle an action in the dropdown menu.
- * @param {!Blockly.MenuItem} menuItem The MenuItem selected within menu.
+ * @param {!MenuItem} menuItem The MenuItem selected within menu.
  * @private
  */
-Blockly.FieldDropdown.prototype.handleMenuActionEvent_ = function(menuItem) {
-  Blockly.DropDownDiv.hideIfOwner(this, true);
-  this.onItemSelected_(/** @type {!Blockly.Menu} */ (this.menu_), menuItem);
+FieldDropdown.prototype.handleMenuActionEvent_ = function(menuItem) {
+  DropDownDiv.hideIfOwner(this, true);
+  this.onItemSelected_(/** @type {!Menu} */ (this.menu_), menuItem);
 };
 
 /**
  * Handle the selection of an item in the dropdown menu.
- * @param {!Blockly.Menu} menu The Menu component clicked.
- * @param {!Blockly.MenuItem} menuItem The MenuItem selected within menu.
+ * @param {!Menu} menu The Menu component clicked.
+ * @param {!MenuItem} menuItem The MenuItem selected within menu.
  * @protected
  */
-Blockly.FieldDropdown.prototype.onItemSelected_ = function(menu, menuItem) {
+FieldDropdown.prototype.onItemSelected_ = function(menu, menuItem) {
   this.setValue(menuItem.getValue());
 };
 
@@ -399,21 +394,21 @@ Blockly.FieldDropdown.prototype.onItemSelected_ = function(menu, menuItem) {
  * Create prefix and/or suffix labels.
  * @private
  */
-Blockly.FieldDropdown.prototype.trimOptions_ = function() {
-  var options = this.menuGenerator_;
+FieldDropdown.prototype.trimOptions_ = function() {
+  const options = this.menuGenerator_;
   if (!Array.isArray(options)) {
     return;
   }
-  var hasImages = false;
+  let hasImages = false;
 
   // Localize label text and image alt text.
-  for (var i = 0; i < options.length; i++) {
-    var label = options[i][0];
+  for (let i = 0; i < options.length; i++) {
+    const label = options[i][0];
     if (typeof label == 'string') {
-      options[i][0] = Blockly.utils.replaceMessageReferences(label);
+      options[i][0] = replaceMessageReferences(label);
     } else {
       if (label.alt != null) {
-        options[i][0].alt = Blockly.utils.replaceMessageReferences(label.alt);
+        options[i][0].alt = replaceMessageReferences(label.alt);
       }
       hasImages = true;
     }
@@ -421,13 +416,13 @@ Blockly.FieldDropdown.prototype.trimOptions_ = function() {
   if (hasImages || options.length < 2) {
     return;  // Do nothing if too few items or at least one label is an image.
   }
-  var strings = [];
-  for (var i = 0; i < options.length; i++) {
+  const strings = [];
+  for (let i = 0; i < options.length; i++) {
     strings.push(options[i][0]);
   }
-  var shortest = Blockly.utils.string.shortestStringLength(strings);
-  var prefixLength = Blockly.utils.string.commonWordPrefix(strings, shortest);
-  var suffixLength = Blockly.utils.string.commonWordSuffix(strings, shortest);
+  const shortest = shortestStringLength(strings);
+  const prefixLength = commonWordPrefix(strings, shortest);
+  const suffixLength = commonWordSuffix(strings, shortest);
   if (!prefixLength && !suffixLength) {
     return;
   }
@@ -442,8 +437,8 @@ Blockly.FieldDropdown.prototype.trimOptions_ = function() {
     this.suffixField = strings[0].substr(1 - suffixLength);
   }
 
-  this.menuGenerator_ = Blockly.FieldDropdown.applyTrim_(options, prefixLength,
-      suffixLength);
+  this.menuGenerator_ =
+      FieldDropdown.applyTrim_(options, prefixLength, suffixLength);
 };
 
 /**
@@ -455,13 +450,12 @@ Blockly.FieldDropdown.prototype.trimOptions_ = function() {
  * @param {number} suffixLength The length of the common suffix
  * @return {!Array<!Array>} A new array with all of the option text trimmed.
  */
-Blockly.FieldDropdown.applyTrim_ = function(options,
-    prefixLength, suffixLength) {
-  var newOptions = [];
+FieldDropdown.applyTrim_ = function(options, prefixLength, suffixLength) {
+  const newOptions = [];
   // Remove the prefix and suffix from the options.
-  for (var i = 0; i < options.length; i++) {
-    var text = options[i][0];
-    var value = options[i][1];
+  for (let i = 0; i < options.length; i++) {
+    let text = options[i][0];
+    const value = options[i][1];
     text = text.substring(prefixLength, text.length - suffixLength);
     newOptions[i] = [text, value];
   }
@@ -472,7 +466,7 @@ Blockly.FieldDropdown.applyTrim_ = function(options,
  * @return {boolean} True if the option list is generated by a function.
  *     Otherwise false.
  */
-Blockly.FieldDropdown.prototype.isOptionListDynamic = function() {
+FieldDropdown.prototype.isOptionListDynamic = function() {
   return typeof this.menuGenerator_ == 'function';
 };
 
@@ -484,11 +478,11 @@ Blockly.FieldDropdown.prototype.isOptionListDynamic = function() {
  *     (human-readable text or image, language-neutral name).
  * @throws {TypeError} If generated options are incorrectly structured.
  */
-Blockly.FieldDropdown.prototype.getOptions = function(opt_useCache) {
+FieldDropdown.prototype.getOptions = function(opt_useCache) {
   if (this.isOptionListDynamic()) {
     if (!this.generatedOptions_ || !opt_useCache) {
       this.generatedOptions_ = this.menuGenerator_.call(this);
-      Blockly.FieldDropdown.validateOptions_(this.generatedOptions_);
+      validateOptions(this.generatedOptions_);
     }
     return this.generatedOptions_;
   }
@@ -501,10 +495,10 @@ Blockly.FieldDropdown.prototype.getOptions = function(opt_useCache) {
  * @return {?string} A valid language-neutral option, or null if invalid.
  * @protected
  */
-Blockly.FieldDropdown.prototype.doClassValidation_ = function(opt_newValue) {
-  var isValueValid = false;
-  var options = this.getOptions(true);
-  for (var i = 0, option; (option = options[i]); i++) {
+FieldDropdown.prototype.doClassValidation_ = function(opt_newValue) {
+  let isValueValid = false;
+  const options = this.getOptions(true);
+  for (let i = 0, option; (option = options[i]); i++) {
     // Options are tuples of human-readable text and language-neutral values.
     if (option[1] == opt_newValue) {
       isValueValid = true;
@@ -513,9 +507,10 @@ Blockly.FieldDropdown.prototype.doClassValidation_ = function(opt_newValue) {
   }
   if (!isValueValid) {
     if (this.sourceBlock_) {
-      console.warn('Cannot set the dropdown\'s value to an unavailable option.' +
-        ' Block type: ' + this.sourceBlock_.type + ', Field name: ' + this.name +
-        ', Value: ' + opt_newValue);
+      console.warn(
+          'Cannot set the dropdown\'s value to an unavailable option.' +
+          ' Block type: ' + this.sourceBlock_.type +
+          ', Field name: ' + this.name + ', Value: ' + opt_newValue);
     }
     return null;
   }
@@ -528,10 +523,10 @@ Blockly.FieldDropdown.prototype.doClassValidation_ = function(opt_newValue) {
  * that this is one of the valid dropdown options.
  * @protected
  */
-Blockly.FieldDropdown.prototype.doValueUpdate_ = function(newValue) {
-  Blockly.FieldDropdown.superClass_.doValueUpdate_.call(this, newValue);
-  var options = this.getOptions(true);
-  for (var i = 0, option; (option = options[i]); i++) {
+FieldDropdown.prototype.doValueUpdate_ = function(newValue) {
+  FieldDropdown.superClass_.doValueUpdate_.call(this, newValue);
+  const options = this.getOptions(true);
+  for (let i = 0, option; (option = options[i]); i++) {
     if (option[1] == this.value_) {
       this.selectedOption_ = option;
     }
@@ -542,13 +537,13 @@ Blockly.FieldDropdown.prototype.doValueUpdate_ = function(newValue) {
  * Updates the dropdown arrow to match the colour/style of the block.
  * @package
  */
-Blockly.FieldDropdown.prototype.applyColour = function() {
+FieldDropdown.prototype.applyColour = function() {
   if (this.borderRect_) {
-    this.borderRect_.setAttribute('stroke',
-        this.sourceBlock_.style.colourTertiary);
+    this.borderRect_.setAttribute(
+        'stroke', this.sourceBlock_.style.colourTertiary);
     if (this.menu_) {
-      this.borderRect_.setAttribute('fill',
-          this.sourceBlock_.style.colourTertiary);
+      this.borderRect_.setAttribute(
+          'fill', this.sourceBlock_.style.colourTertiary);
     } else {
       this.borderRect_.setAttribute('fill', 'transparent');
     }
@@ -567,16 +562,16 @@ Blockly.FieldDropdown.prototype.applyColour = function() {
  * Draws the border with the correct width.
  * @protected
  */
-Blockly.FieldDropdown.prototype.render_ = function() {
+FieldDropdown.prototype.render_ = function() {
   // Hide both elements.
   this.textContent_.nodeValue = '';
   this.imageElement_.style.display = 'none';
 
   // Show correct element.
-  var option = this.selectedOption_ && this.selectedOption_[0];
+  const option = this.selectedOption_ && this.selectedOption_[0];
   if (option && typeof option == 'object') {
     this.renderSelectedImage_(
-        /** @type {!Blockly.FieldDropdown.ImageProperties} */ (option));
+        /** @type {!FieldDropdown.ImageProperties} */ (option));
   } else {
     this.renderSelectedText_();
   }
@@ -586,32 +581,33 @@ Blockly.FieldDropdown.prototype.render_ = function() {
 
 /**
  * Renders the selected option, which must be an image.
- * @param {!Blockly.FieldDropdown.ImageProperties} imageJson Selected
+ * @param {!FieldDropdown.ImageProperties} imageJson Selected
  *   option that must be an image.
  * @private
  */
-Blockly.FieldDropdown.prototype.renderSelectedImage_ = function(imageJson) {
+FieldDropdown.prototype.renderSelectedImage_ = function(imageJson) {
   this.imageElement_.style.display = '';
-  this.imageElement_.setAttributeNS(
-      Blockly.utils.dom.XLINK_NS, 'xlink:href', imageJson.src);
+  this.imageElement_.setAttributeNS(dom.XLINK_NS, 'xlink:href', imageJson.src);
   this.imageElement_.setAttribute('height', imageJson.height);
   this.imageElement_.setAttribute('width', imageJson.width);
 
-  var imageHeight = Number(imageJson.height);
-  var imageWidth = Number(imageJson.width);
+  const imageHeight = Number(imageJson.height);
+  const imageWidth = Number(imageJson.width);
 
   // Height and width include the border rect.
-  var hasBorder = !!this.borderRect_;
-  var height = Math.max(
+  const hasBorder = !!this.borderRect_;
+  const height = Math.max(
       hasBorder ? this.getConstants().FIELD_DROPDOWN_BORDER_RECT_HEIGHT : 0,
-      imageHeight + Blockly.FieldDropdown.IMAGE_Y_PADDING);
-  var xPadding = hasBorder ? this.getConstants().FIELD_BORDER_RECT_X_PADDING : 0;
-  var arrowWidth = 0;
+      imageHeight + IMAGE_Y_PADDING);
+  const xPadding =
+      hasBorder ? this.getConstants().FIELD_BORDER_RECT_X_PADDING : 0;
+  let arrowWidth = 0;
   if (this.svgArrow_) {
-    arrowWidth = this.positionSVGArrow_(imageWidth + xPadding, height / 2 -
-      this.getConstants().FIELD_DROPDOWN_SVG_ARROW_SIZE / 2);
+    arrowWidth = this.positionSVGArrow_(
+        imageWidth + xPadding,
+        height / 2 - this.getConstants().FIELD_DROPDOWN_SVG_ARROW_SIZE / 2);
   } else {
-    arrowWidth = Blockly.utils.dom.getFastTextWidth(
+    arrowWidth = dom.getFastTextWidth(
         /** @type {!SVGTSpanElement} */ (this.arrow_),
         this.getConstants().FIELD_TEXT_FONTSIZE,
         this.getConstants().FIELD_TEXT_FONTWEIGHT,
@@ -620,9 +616,9 @@ Blockly.FieldDropdown.prototype.renderSelectedImage_ = function(imageJson) {
   this.size_.width = imageWidth + arrowWidth + xPadding * 2;
   this.size_.height = height;
 
-  var arrowX = 0;
+  let arrowX = 0;
   if (this.sourceBlock_.RTL) {
-    var imageX = xPadding + arrowWidth;
+    const imageX = xPadding + arrowWidth;
     this.imageElement_.setAttribute('x', imageX);
   } else {
     arrowX = imageWidth + arrowWidth;
@@ -638,27 +634,29 @@ Blockly.FieldDropdown.prototype.renderSelectedImage_ = function(imageJson) {
  * Renders the selected option, which must be text.
  * @private
  */
-Blockly.FieldDropdown.prototype.renderSelectedText_ = function() {
+FieldDropdown.prototype.renderSelectedText_ = function() {
   // Retrieves the selected option to display through getText_.
   this.textContent_.nodeValue = this.getDisplayText_();
-  Blockly.utils.dom.addClass(/** @type {!Element} */ (this.textElement_),
-      'blocklyDropdownText');
+  dom.addClass(
+      /** @type {!Element} */ (this.textElement_), 'blocklyDropdownText');
   this.textElement_.setAttribute('text-anchor', 'start');
 
   // Height and width include the border rect.
-  var hasBorder = !!this.borderRect_;
-  var height = Math.max(
+  const hasBorder = !!this.borderRect_;
+  const height = Math.max(
       hasBorder ? this.getConstants().FIELD_DROPDOWN_BORDER_RECT_HEIGHT : 0,
       this.getConstants().FIELD_TEXT_HEIGHT);
-  var textWidth = Blockly.utils.dom.getFastTextWidth(this.textElement_,
-      this.getConstants().FIELD_TEXT_FONTSIZE,
+  const textWidth = dom.getFastTextWidth(
+      this.textElement_, this.getConstants().FIELD_TEXT_FONTSIZE,
       this.getConstants().FIELD_TEXT_FONTWEIGHT,
       this.getConstants().FIELD_TEXT_FONTFAMILY);
-  var xPadding = hasBorder ? this.getConstants().FIELD_BORDER_RECT_X_PADDING : 0;
-  var arrowWidth = 0;
+  const xPadding =
+      hasBorder ? this.getConstants().FIELD_BORDER_RECT_X_PADDING : 0;
+  let arrowWidth = 0;
   if (this.svgArrow_) {
-    arrowWidth = this.positionSVGArrow_(textWidth + xPadding, height / 2 -
-        this.getConstants().FIELD_DROPDOWN_SVG_ARROW_SIZE / 2);
+    arrowWidth = this.positionSVGArrow_(
+        textWidth + xPadding,
+        height / 2 - this.getConstants().FIELD_DROPDOWN_SVG_ARROW_SIZE / 2);
   }
   this.size_.width = textWidth + arrowWidth + xPadding * 2;
   this.size_.height = height;
@@ -673,17 +671,18 @@ Blockly.FieldDropdown.prototype.renderSelectedText_ = function() {
  * @return {number} Amount of space the arrow is taking up, in px.
  * @private
  */
-Blockly.FieldDropdown.prototype.positionSVGArrow_ = function(x, y) {
+FieldDropdown.prototype.positionSVGArrow_ = function(x, y) {
   if (!this.svgArrow_) {
     return 0;
   }
-  var hasBorder = !!this.borderRect_;
-  var xPadding = hasBorder ? this.getConstants().FIELD_BORDER_RECT_X_PADDING : 0;
-  var textPadding = this.getConstants().FIELD_DROPDOWN_SVG_ARROW_PADDING;
-  var svgArrowSize = this.getConstants().FIELD_DROPDOWN_SVG_ARROW_SIZE;
-  var arrowX = this.sourceBlock_.RTL ? xPadding : x + textPadding;
-  this.svgArrow_.setAttribute('transform',
-      'translate(' + arrowX + ',' + y + ')');
+  const hasBorder = !!this.borderRect_;
+  const xPadding =
+      hasBorder ? this.getConstants().FIELD_BORDER_RECT_X_PADDING : 0;
+  const textPadding = this.getConstants().FIELD_DROPDOWN_SVG_ARROW_PADDING;
+  const svgArrowSize = this.getConstants().FIELD_DROPDOWN_SVG_ARROW_SIZE;
+  const arrowX = this.sourceBlock_.RTL ? xPadding : x + textPadding;
+  this.svgArrow_.setAttribute(
+      'transform', 'translate(' + arrowX + ',' + y + ')');
   return svgArrowSize + textPadding;
 };
 
@@ -695,11 +694,11 @@ Blockly.FieldDropdown.prototype.positionSVGArrow_ = function(x, y) {
  * @protected
  * @override
  */
-Blockly.FieldDropdown.prototype.getText_ = function() {
+FieldDropdown.prototype.getText_ = function() {
   if (!this.selectedOption_) {
     return null;
   }
-  var option = this.selectedOption_[0];
+  const option = this.selectedOption_[0];
   if (typeof option == 'object') {
     return option['alt'];
   }
@@ -710,35 +709,36 @@ Blockly.FieldDropdown.prototype.getText_ = function() {
  * Validates the data structure to be processed as an options list.
  * @param {?} options The proposed dropdown options.
  * @throws {TypeError} If proposed options are incorrectly structured.
- * @private
  */
-Blockly.FieldDropdown.validateOptions_ = function(options) {
+const validateOptions = function(options) {
   if (!Array.isArray(options)) {
     throw TypeError('FieldDropdown options must be an array.');
   }
   if (!options.length) {
     throw TypeError('FieldDropdown options must not be an empty array.');
   }
-  var foundError = false;
-  for (var i = 0; i < options.length; ++i) {
-    var tuple = options[i];
+  let foundError = false;
+  for (let i = 0; i < options.length; ++i) {
+    const tuple = options[i];
     if (!Array.isArray(tuple)) {
       foundError = true;
       console.error(
           'Invalid option[' + i + ']: Each FieldDropdown option must be an ' +
-          'array. Found: ', tuple);
+              'array. Found: ',
+          tuple);
     } else if (typeof tuple[1] != 'string') {
       foundError = true;
       console.error(
           'Invalid option[' + i + ']: Each FieldDropdown option id must be ' +
-          'a string. Found ' + tuple[1] + ' in: ', tuple);
-    } else if (tuple[0] &&
-              (typeof tuple[0] != 'string') &&
-              (typeof tuple[0].src != 'string')) {
+              'a string. Found ' + tuple[1] + ' in: ',
+          tuple);
+    } else if (
+        tuple[0] && (typeof tuple[0] != 'string') &&
+        (typeof tuple[0].src != 'string')) {
       foundError = true;
       console.error(
           'Invalid option[' + i + ']: Each FieldDropdown option must have a ' +
-          'string label or image description. Found' + tuple[0] + ' in: ',
+              'string label or image description. Found' + tuple[0] + ' in: ',
           tuple);
     }
   }
@@ -747,4 +747,6 @@ Blockly.FieldDropdown.validateOptions_ = function(options) {
   }
 };
 
-Blockly.fieldRegistry.register('field_dropdown', Blockly.FieldDropdown);
+fieldRegistry.register('field_dropdown', FieldDropdown);
+
+exports = FieldDropdown;
diff --git a/tests/deps.js b/tests/deps.js
index bfe2639f5..da56c39f2 100644
--- a/tests/deps.js
+++ b/tests/deps.js
@@ -52,7 +52,7 @@ goog.addDependency('../../core/field.js', ['Blockly.Field'], ['Blockly.DropDownD
 goog.addDependency('../../core/field_angle.js', ['Blockly.FieldAngle'], ['Blockly.Css', 'Blockly.DropDownDiv', 'Blockly.FieldTextInput', 'Blockly.WidgetDiv', 'Blockly.browserEvents', 'Blockly.fieldRegistry', 'Blockly.utils.KeyCodes', 'Blockly.utils.Svg', 'Blockly.utils.dom', 'Blockly.utils.math', 'Blockly.utils.object', 'Blockly.utils.userAgent']);
 goog.addDependency('../../core/field_checkbox.js', ['Blockly.FieldCheckbox'], ['Blockly.Events.BlockChange', 'Blockly.Field', 'Blockly.fieldRegistry', 'Blockly.utils.dom', 'Blockly.utils.object'], {'lang': 'es6', 'module': 'goog'});
 goog.addDependency('../../core/field_colour.js', ['Blockly.FieldColour'], ['Blockly.Css', 'Blockly.DropDownDiv', 'Blockly.Events.BlockChange', 'Blockly.Field', 'Blockly.browserEvents', 'Blockly.fieldRegistry', 'Blockly.utils.IdGenerator', 'Blockly.utils.KeyCodes', 'Blockly.utils.Size', 'Blockly.utils.aria', 'Blockly.utils.colour', 'Blockly.utils.dom', 'Blockly.utils.object'], {'lang': 'es6', 'module': 'goog'});
-goog.addDependency('../../core/field_dropdown.js', ['Blockly.FieldDropdown'], ['Blockly.DropDownDiv', 'Blockly.Field', 'Blockly.Menu', 'Blockly.MenuItem', 'Blockly.fieldRegistry', 'Blockly.utils', 'Blockly.utils.Coordinate', 'Blockly.utils.Svg', 'Blockly.utils.aria', 'Blockly.utils.dom', 'Blockly.utils.object', 'Blockly.utils.string', 'Blockly.utils.userAgent']);
+goog.addDependency('../../core/field_dropdown.js', ['Blockly.FieldDropdown'], ['Blockly.DropDownDiv', 'Blockly.Field', 'Blockly.Menu', 'Blockly.MenuItem', 'Blockly.fieldRegistry', 'Blockly.utils', 'Blockly.utils.Coordinate', 'Blockly.utils.Svg', 'Blockly.utils.aria', 'Blockly.utils.dom', 'Blockly.utils.object', 'Blockly.utils.string', 'Blockly.utils.userAgent'], {'lang': 'es6', 'module': 'goog'});
 goog.addDependency('../../core/field_image.js', ['Blockly.FieldImage'], ['Blockly.Field', 'Blockly.fieldRegistry', 'Blockly.utils', 'Blockly.utils.Size', 'Blockly.utils.Svg', 'Blockly.utils.dom', 'Blockly.utils.object']);
 goog.addDependency('../../core/field_label.js', ['Blockly.FieldLabel'], ['Blockly.Field', 'Blockly.fieldRegistry', 'Blockly.utils', 'Blockly.utils.dom', 'Blockly.utils.object']);
 goog.addDependency('../../core/field_label_serializable.js', ['Blockly.FieldLabelSerializable'], ['Blockly.FieldLabel', 'Blockly.fieldRegistry', 'Blockly.utils', 'Blockly.utils.object']);