Review updates

core/field.js

@@ -55,6 +55,7 @@ goog.requireType('Blockly.WorkspaceSvg');
  *    the individual field's documentation for a list of properties this
  *    parameter supports.
  * @constructor
+ * @abstract
  * @implements {Blockly.IASTNodeLocationSvg}
  * @implements {Blockly.IASTNodeLocationWithBlock}
  * @implements {Blockly.IKeyboardAccessible}
@@ -154,14 +155,6 @@ Blockly.Field = function(value, opt_validator, opt_config) {
   opt_validator && this.setValidator(opt_validator);
 };
 
-/**
- * Construct a Field from a JSON arg object.
- * @param {!Object} _options A JSON object with options.
- */
-Blockly.Field.fromJson = function(_options) {
-  throw Error("Field subclass doesn't define fromJson");
-};
-
 /**
  * The default value for this field.
  * @type {*}

core/field_dropdown.js

@@ -151,7 +151,7 @@ Blockly.FieldDropdown.ImageProperties;
 Blockly.FieldDropdown.fromJson = function(options) {
   // `this` might be a subclass of FieldDropdown if that class doesn't override
   // the static fromJson method.
-  return new Blockly.FieldDropdown(options['options'], undefined, options);
+  return new this(options['options'], undefined, options);
 };
 
 /**

core/field_multilineinput.js

@@ -89,8 +89,8 @@ Blockly.FieldMultilineInput.prototype.configure_ = function(config) {
  */
 Blockly.FieldMultilineInput.fromJson = function(options) {
   var text = Blockly.utils.replaceMessageReferences(options['text']);
-  // `this` might be a subclass of FieldLabel if that class doesn't override
-  // the static fromJson method.
+  // `this` might be a subclass of FieldMultilineInput if that class doesn't
+  // override the static fromJson method.
   return new this(text, undefined, options);
 };