diff --git a/core/toolbox/category.js b/core/toolbox/category.js
index d035c13d1..3613b5a75 100644
--- a/core/toolbox/category.js
+++ b/core/toolbox/category.js
@@ -10,8 +10,11 @@
  */
 'use strict';
 
-goog.provide('Blockly.ToolboxCategory');
+goog.module('Blockly.ToolboxCategory');
+goog.module.declareLegacyNamespace();
 
+goog.require('Blockly');
+goog.require('Blockly.Css');
 goog.require('Blockly.ISelectableToolboxItem');
 goog.require('Blockly.registry');
 goog.require('Blockly.ToolboxItem');
@@ -36,8 +39,8 @@ goog.requireType('Blockly.IToolbox');
  * @extends {Blockly.ToolboxItem}
  * @implements {Blockly.ISelectableToolboxItem}
  */
-Blockly.ToolboxCategory = function(categoryDef, toolbox, opt_parent) {
-  Blockly.ToolboxCategory.superClass_.constructor.call(
+const ToolboxCategory = function(categoryDef, toolbox, opt_parent) {
+  ToolboxCategory.superClass_.constructor.call(
       this, categoryDef, toolbox, opt_parent);
 
   /**
@@ -91,7 +94,7 @@ Blockly.ToolboxCategory = function(categoryDef, toolbox, opt_parent) {
 
   /**
    * All the css class names that are used to create a category.
-   * @type {!Blockly.ToolboxCategory.CssConfig}
+   * @type {!ToolboxCategory.CssConfig}
    * @protected
    */
   this.cssConfig_ = this.makeDefaultCssConfig_();
@@ -123,7 +126,7 @@ Blockly.ToolboxCategory = function(categoryDef, toolbox, opt_parent) {
   this.parseContents_(categoryDef);
 };
 
-Blockly.utils.object.inherits(Blockly.ToolboxCategory, Blockly.ToolboxItem);
+Blockly.utils.object.inherits(ToolboxCategory, Blockly.ToolboxItem);
 
 /**
  * All the CSS class names that are used to create a category.
@@ -138,40 +141,40 @@ Blockly.utils.object.inherits(Blockly.ToolboxCategory, Blockly.ToolboxItem);
  *            closedicon:(string|undefined)
  *          }}
  */
-Blockly.ToolboxCategory.CssConfig;
+ToolboxCategory.CssConfig;
 
 /**
  * Name used for registering a toolbox category.
  * @const {string}
  */
-Blockly.ToolboxCategory.registrationName = 'category';
+ToolboxCategory.registrationName = 'category';
 
 /**
  * The number of pixels to move the category over at each nested level.
  * @type {number}
  */
-Blockly.ToolboxCategory.nestedPadding = 19;
+ToolboxCategory.nestedPadding = 19;
 
 /**
  * The width in pixels of the strip of colour next to each category.
  * @type {number}
  */
-Blockly.ToolboxCategory.borderWidth = 8;
+ToolboxCategory.borderWidth = 8;
 
 /**
  * The default colour of the category. This is used as the background colour of
  * the category when it is selected.
  * @type {string}
  */
-Blockly.ToolboxCategory.defaultBackgroundColour = '#57e';
+ToolboxCategory.defaultBackgroundColour = '#57e';
 
 /**
  * Creates an object holding the default classes for a category.
- * @return {!Blockly.ToolboxCategory.CssConfig} The configuration object holding
+ * @return {!ToolboxCategory.CssConfig} The configuration object holding
  *    all the CSS classes for a category.
  * @protected
  */
-Blockly.ToolboxCategory.prototype.makeDefaultCssConfig_ = function() {
+ToolboxCategory.prototype.makeDefaultCssConfig_ = function() {
   return {
     'container': 'blocklyToolboxCategory',
     'row': 'blocklyTreeRow',
@@ -192,7 +195,7 @@ Blockly.ToolboxCategory.prototype.makeDefaultCssConfig_ = function() {
  *     to create a category.
  * @protected
  */
-Blockly.ToolboxCategory.prototype.parseContents_ = function(categoryDef) {
+ToolboxCategory.prototype.parseContents_ = function(categoryDef) {
   const contents = categoryDef['contents'];
 
   if (categoryDef['custom']) {
@@ -209,7 +212,7 @@ Blockly.ToolboxCategory.prototype.parseContents_ = function(categoryDef) {
 /**
  * @override
  */
-Blockly.ToolboxCategory.prototype.init = function() {
+ToolboxCategory.prototype.init = function() {
   this.createDom_();
   if (this.toolboxItemDef_['hidden'] == 'true') {
     this.hide();
@@ -221,7 +224,7 @@ Blockly.ToolboxCategory.prototype.init = function() {
  * @return {!Element} The parent element for the category.
  * @protected
  */
-Blockly.ToolboxCategory.prototype.createDom_ = function() {
+ToolboxCategory.prototype.createDom_ = function() {
   this.htmlDiv_ = this.createContainer_();
   Blockly.utils.aria.setRole(this.htmlDiv_, Blockly.utils.aria.Role.TREEITEM);
   Blockly.utils.aria.setState(/** @type {!Element} */ (this.htmlDiv_),
@@ -256,7 +259,7 @@ Blockly.ToolboxCategory.prototype.createDom_ = function() {
  * @return {!Element} The div that holds the icon and the label.
  * @protected
  */
-Blockly.ToolboxCategory.prototype.createContainer_ = function() {
+ToolboxCategory.prototype.createContainer_ = function() {
   const container = document.createElement('div');
   Blockly.utils.dom.addClass(container, this.cssConfig_['container']);
   return container;
@@ -268,10 +271,10 @@ Blockly.ToolboxCategory.prototype.createContainer_ = function() {
  * @return {!Element} The div that holds the contents container.
  * @protected
  */
-Blockly.ToolboxCategory.prototype.createRowContainer_ = function() {
+ToolboxCategory.prototype.createRowContainer_ = function() {
   const rowDiv = document.createElement('div');
   Blockly.utils.dom.addClass(rowDiv, this.cssConfig_['row']);
-  let nestedPadding = Blockly.ToolboxCategory.nestedPadding * this.getLevel();
+  let nestedPadding = ToolboxCategory.nestedPadding * this.getLevel();
   nestedPadding = nestedPadding.toString() + 'px';
   this.workspace_.RTL ? rowDiv.style.paddingRight = nestedPadding :
       rowDiv.style.paddingLeft = nestedPadding;
@@ -284,7 +287,7 @@ Blockly.ToolboxCategory.prototype.createRowContainer_ = function() {
  * @return {!Element} The div that holds the icon and the label.
  * @protected
  */
-Blockly.ToolboxCategory.prototype.createRowContentsContainer_ = function() {
+ToolboxCategory.prototype.createRowContentsContainer_ = function() {
   const contentsContainer = document.createElement('div');
   Blockly.utils.dom.addClass(contentsContainer, this.cssConfig_['rowcontentcontainer']);
   return contentsContainer;
@@ -295,7 +298,7 @@ Blockly.ToolboxCategory.prototype.createRowContentsContainer_ = function() {
  * @return {!Element} The span that holds the category icon.
  * @protected
  */
-Blockly.ToolboxCategory.prototype.createIconDom_ = function() {
+ToolboxCategory.prototype.createIconDom_ = function() {
   const toolboxIcon = document.createElement('span');
   if (!this.parentToolbox_.isHorizontal()) {
     Blockly.utils.dom.addClass(toolboxIcon, this.cssConfig_['icon']);
@@ -312,7 +315,7 @@ Blockly.ToolboxCategory.prototype.createIconDom_ = function() {
  * @return {!Element} The span that holds the category label.
  * @protected
  */
-Blockly.ToolboxCategory.prototype.createLabelDom_ = function(name) {
+ToolboxCategory.prototype.createLabelDom_ = function(name) {
   const toolboxLabel = document.createElement('span');
   toolboxLabel.setAttribute('id', this.getId() + '.label');
   toolboxLabel.textContent = name;
@@ -324,7 +327,7 @@ Blockly.ToolboxCategory.prototype.createLabelDom_ = function(name) {
  * Updates the colour for this category.
  * @public
  */
-Blockly.ToolboxCategory.prototype.refreshTheme = function() {
+ToolboxCategory.prototype.refreshTheme = function() {
   this.colour_ = this.getColour_(/** @type {Blockly.utils.toolbox.CategoryInfo} **/
       (this.toolboxItemDef_));
   this.addColourBorder_(this.colour_);
@@ -335,9 +338,9 @@ Blockly.ToolboxCategory.prototype.refreshTheme = function() {
  * @param {string} colour The category colour.
  * @protected
  */
-Blockly.ToolboxCategory.prototype.addColourBorder_ = function(colour) {
+ToolboxCategory.prototype.addColourBorder_ = function(colour) {
   if (colour) {
-    const border = Blockly.ToolboxCategory.borderWidth + 'px solid ' +
+    const border = ToolboxCategory.borderWidth + 'px solid ' +
         (colour || '#ddd');
     if (this.workspace_.RTL) {
       this.rowDiv_.style.borderRight = border;
@@ -354,7 +357,7 @@ Blockly.ToolboxCategory.prototype.addColourBorder_ = function(colour) {
  * @return {string} The hex colour for the category.
  * @protected
  */
-Blockly.ToolboxCategory.prototype.getColour_ = function(categoryDef) {
+ToolboxCategory.prototype.getColour_ = function(categoryDef) {
   const styleName = categoryDef['categorystyle']
       || categoryDef['categoryStyle'];
   const colour = categoryDef['colour'];
@@ -377,7 +380,7 @@ Blockly.ToolboxCategory.prototype.getColour_ = function(categoryDef) {
  * @return {string} The hex colour for the category.
  * @private
  */
-Blockly.ToolboxCategory.prototype.getColourfromStyle_ = function(styleName) {
+ToolboxCategory.prototype.getColourfromStyle_ = function(styleName) {
   const theme = this.workspace_.getTheme();
   if (styleName && theme) {
     const style = theme.categoryStyles[styleName];
@@ -398,7 +401,7 @@ Blockly.ToolboxCategory.prototype.getColourfromStyle_ = function(styleName) {
  * @return {!Element} The HTML element that receives clicks.
  * @public
  */
-Blockly.ToolboxCategory.prototype.getClickTarget = function() {
+ToolboxCategory.prototype.getClickTarget = function() {
   return /** @type {!Element} */(this.rowDiv_);
 };
 
@@ -409,7 +412,7 @@ Blockly.ToolboxCategory.prototype.getClickTarget = function() {
  * @return {string} The hex colour for the category.
  * @private
  */
-Blockly.ToolboxCategory.prototype.parseColour_ = function(colourValue) {
+ToolboxCategory.prototype.parseColour_ = function(colourValue) {
   // Decode the colour for any potential message references
   // (eg. `%{BKY_MATH_HUE}`).
   const colour = Blockly.utils.replaceMessageReferences(colourValue);
@@ -438,7 +441,7 @@ Blockly.ToolboxCategory.prototype.parseColour_ = function(colourValue) {
  * @param {?Element} iconDiv The div that holds the icon.
  * @protected
  */
-Blockly.ToolboxCategory.prototype.openIcon_ = function(iconDiv) {
+ToolboxCategory.prototype.openIcon_ = function(iconDiv) {
   if (!iconDiv) {
     return;
   }
@@ -451,7 +454,7 @@ Blockly.ToolboxCategory.prototype.openIcon_ = function(iconDiv) {
  * @param {?Element} iconDiv The div that holds the icon.
  * @protected
  */
-Blockly.ToolboxCategory.prototype.closeIcon_ = function(iconDiv) {
+ToolboxCategory.prototype.closeIcon_ = function(iconDiv) {
   if (!iconDiv) {
     return;
   }
@@ -465,7 +468,7 @@ Blockly.ToolboxCategory.prototype.closeIcon_ = function(iconDiv) {
  * @param {boolean} isVisible True if category should be visible.
  * @protected
  */
-Blockly.ToolboxCategory.prototype.setVisible_ = function(isVisible) {
+ToolboxCategory.prototype.setVisible_ = function(isVisible) {
   this.htmlDiv_.style.display = isVisible ? 'block' : 'none';
   this.isHidden_ = !isVisible;
 
@@ -477,7 +480,7 @@ Blockly.ToolboxCategory.prototype.setVisible_ = function(isVisible) {
 /**
  * Hide the category.
  */
-Blockly.ToolboxCategory.prototype.hide = function() {
+ToolboxCategory.prototype.hide = function() {
   this.setVisible_(false);
 };
 
@@ -485,7 +488,7 @@ Blockly.ToolboxCategory.prototype.hide = function() {
  * Show the category. Category will only appear if its parent category is also
  * expanded.
  */
-Blockly.ToolboxCategory.prototype.show = function() {
+ToolboxCategory.prototype.show = function() {
   this.setVisible_(true);
 };
 
@@ -495,7 +498,7 @@ Blockly.ToolboxCategory.prototype.show = function() {
  * @return {boolean} True if the category is visible, false otherwise.
  * @public
  */
-Blockly.ToolboxCategory.prototype.isVisible = function() {
+ToolboxCategory.prototype.isVisible = function() {
   return !this.isHidden_ && this.allAncestorsExpanded_();
 };
 
@@ -504,7 +507,7 @@ Blockly.ToolboxCategory.prototype.isVisible = function() {
  * @return {boolean} True only if every ancestor is expanded
  * @protected
  */
-Blockly.ToolboxCategory.prototype.allAncestorsExpanded_ = function() {
+ToolboxCategory.prototype.allAncestorsExpanded_ = function() {
   let category = this;
   while (category.getParent()) {
     category = category.getParent();
@@ -518,7 +521,7 @@ Blockly.ToolboxCategory.prototype.allAncestorsExpanded_ = function() {
 /**
  * @override
  */
-Blockly.ToolboxCategory.prototype.isSelectable = function() {
+ToolboxCategory.prototype.isSelectable = function() {
   return this.isVisible() && !this.isDisabled_;
 };
 
@@ -527,7 +530,7 @@ Blockly.ToolboxCategory.prototype.isSelectable = function() {
  * @param {!Event} _e Click event to handle.
  * @public
  */
-Blockly.ToolboxCategory.prototype.onClick = function(_e) {
+ToolboxCategory.prototype.onClick = function(_e) {
   // No-op
 };
 
@@ -537,10 +540,10 @@ Blockly.ToolboxCategory.prototype.onClick = function(_e) {
  *     otherwise.
  * @public
  */
-Blockly.ToolboxCategory.prototype.setSelected = function(isSelected) {
+ToolboxCategory.prototype.setSelected = function(isSelected) {
   if (isSelected) {
     const defaultColour = this.parseColour_(
-        Blockly.ToolboxCategory.defaultBackgroundColour);
+        ToolboxCategory.defaultBackgroundColour);
     this.rowDiv_.style.backgroundColor = this.colour_ || defaultColour;
     Blockly.utils.dom.addClass(this.rowDiv_, this.cssConfig_['selected']);
   } else {
@@ -555,7 +558,7 @@ Blockly.ToolboxCategory.prototype.setSelected = function(isSelected) {
  * Sets whether the category is disabled.
  * @param {boolean} isDisabled True to disable the category, false otherwise.
  */
-Blockly.ToolboxCategory.prototype.setDisabled = function(isDisabled) {
+ToolboxCategory.prototype.setDisabled = function(isDisabled) {
   this.isDisabled_ = isDisabled;
   this.getDiv().setAttribute('disabled', isDisabled);
   isDisabled ? this.getDiv().setAttribute('disabled', 'true') :
@@ -567,21 +570,21 @@ Blockly.ToolboxCategory.prototype.setDisabled = function(isDisabled) {
  * @return {string} The name of the toolbox item.
  * @public
  */
-Blockly.ToolboxCategory.prototype.getName = function() {
+ToolboxCategory.prototype.getName = function() {
   return this.name_;
 };
 
 /**
  * @override
  */
-Blockly.ToolboxCategory.prototype.getParent = function() {
+ToolboxCategory.prototype.getParent = function() {
   return this.parent_;
 };
 
 /**
  * @override
  */
-Blockly.ToolboxCategory.prototype.getDiv = function() {
+ToolboxCategory.prototype.getDiv = function() {
   return this.htmlDiv_;
 };
 
@@ -592,7 +595,7 @@ Blockly.ToolboxCategory.prototype.getDiv = function() {
  *     of items to be displayed in the flyout.
  * @public
  */
-Blockly.ToolboxCategory.prototype.getContents = function() {
+ToolboxCategory.prototype.getContents = function() {
   return this.flyoutItems_;
 };
 
@@ -605,7 +608,7 @@ Blockly.ToolboxCategory.prototype.getContents = function() {
  *     dynamic category.
  * @public
  */
-Blockly.ToolboxCategory.prototype.updateFlyoutContents = function(contents) {
+ToolboxCategory.prototype.updateFlyoutContents = function(contents) {
   this.flyoutItems_ = [];
 
   if (typeof contents == 'string') {
@@ -623,7 +626,7 @@ Blockly.ToolboxCategory.prototype.updateFlyoutContents = function(contents) {
 /**
  * @override
  */
-Blockly.ToolboxCategory.prototype.dispose = function() {
+ToolboxCategory.prototype.dispose = function() {
   Blockly.utils.dom.removeNode(this.htmlDiv_);
 };
 
@@ -707,4 +710,6 @@ Blockly.Css.register([
 ]);
 
 Blockly.registry.register(Blockly.registry.Type.TOOLBOX_ITEM,
-    Blockly.ToolboxCategory.registrationName, Blockly.ToolboxCategory);
+    ToolboxCategory.registrationName, ToolboxCategory);
+
+exports = ToolboxCategory;
diff --git a/tests/deps.js b/tests/deps.js
index c71382196..a563d81e7 100644
--- a/tests/deps.js
+++ b/tests/deps.js
@@ -169,7 +169,7 @@ goog.addDependency('../../core/theme.js', ['Blockly.Theme'], ['Blockly.registry'
 goog.addDependency('../../core/theme/classic.js', ['Blockly.Themes.Classic'], ['Blockly.Theme']);
 goog.addDependency('../../core/theme/zelos.js', ['Blockly.Themes.Zelos'], ['Blockly.Theme']);
 goog.addDependency('../../core/theme_manager.js', ['Blockly.ThemeManager'], ['Blockly.utils.dom'], {'lang': 'es6', 'module': 'goog'});
-goog.addDependency('../../core/toolbox/category.js', ['Blockly.ToolboxCategory'], ['Blockly.ISelectableToolboxItem', 'Blockly.ToolboxItem', 'Blockly.registry', 'Blockly.utils', 'Blockly.utils.aria', 'Blockly.utils.dom', 'Blockly.utils.object', 'Blockly.utils.toolbox'], {'lang': 'es5'});
+goog.addDependency('../../core/toolbox/category.js', ['Blockly.ToolboxCategory'], ['Blockly', 'Blockly.Css', 'Blockly.ISelectableToolboxItem', 'Blockly.ToolboxItem', 'Blockly.registry', 'Blockly.utils', 'Blockly.utils.aria', 'Blockly.utils.dom', 'Blockly.utils.object', 'Blockly.utils.toolbox'], {'lang': 'es6', 'module': 'goog'});
 goog.addDependency('../../core/toolbox/collapsible_category.js', ['Blockly.CollapsibleToolboxCategory'], ['Blockly.ICollapsibleToolboxItem', 'Blockly.ToolboxCategory', 'Blockly.ToolboxSeparator', 'Blockly.registry', 'Blockly.utils.aria', 'Blockly.utils.dom', 'Blockly.utils.object', 'Blockly.utils.toolbox'], {'lang': 'es6', 'module': 'goog'});
 goog.addDependency('../../core/toolbox/separator.js', ['Blockly.ToolboxSeparator'], ['Blockly.Css', 'Blockly.ToolboxItem', 'Blockly.registry', 'Blockly.utils.dom', 'Blockly.utils.object'], {'lang': 'es6', 'module': 'goog'});
 goog.addDependency('../../core/toolbox/toolbox.js', ['Blockly.Toolbox'], ['Blockly.BlockSvg', 'Blockly.CollapsibleToolboxCategory', 'Blockly.ComponentManager', 'Blockly.Css', 'Blockly.DeleteArea', 'Blockly.Events', 'Blockly.Events.ToolboxItemSelect', 'Blockly.IAutoHideable', 'Blockly.IKeyboardAccessible', 'Blockly.IStyleable', 'Blockly.IToolbox', 'Blockly.Options', 'Blockly.Touch', 'Blockly.browserEvents', 'Blockly.constants', 'Blockly.registry', 'Blockly.utils', 'Blockly.utils.Rect', 'Blockly.utils.aria', 'Blockly.utils.dom', 'Blockly.utils.toolbox'], {'lang': 'es5'});