Added isSerializable function and SERIALIZABLE property to fields.

core/field_checkbox.js

@@ -58,6 +58,14 @@ Blockly.FieldCheckbox.fromJson = function(options) {
   return new Blockly.FieldCheckbox(options['checked'] ? 'TRUE' : 'FALSE');
 };
 
+/**
+ * Serializable fields are saved by the XML renderer, non-serializable fields
+ * are not. Editable fields should also be serializable.
+ * @type {boolean}
+ * @const
+ */
+Blockly.FieldCheckbox.prototype.SERIALIZABLE = true;
+
 /**
  * Character for the checkmark.
  */