diff --git a/blocks/logic.js b/blocks/logic.js
index 051c46fdf..ccc1c0335 100644
--- a/blocks/logic.js
+++ b/blocks/logic.js
@@ -459,7 +459,7 @@ Blockly.Constants.Logic.CONTROLS_IF_MUTATOR_MIXIN = {
 };
 
 Blockly.Extensions.registerMutator('controls_if_mutator',
-    Blockly.Constants.Logic.CONTROLS_IF_MUTATOR_MIXIN,
+    Blockly.Constants.Logic.CONTROLS_IF_MUTATOR_MIXIN, null,
     ['controls_if_elseif', 'controls_if_else']);
 /**
  * "controls_if" extension function. Adds mutator, shape updating methods, and
diff --git a/blocks/math.js b/blocks/math.js
index c8e6855b0..f88d19bdb 100644
--- a/blocks/math.js
+++ b/blocks/math.js
@@ -207,7 +207,7 @@ Blockly.defineBlocksWithJsonArray([  // BEGIN JSON EXTRACT
     "output": "Boolean",
     "colour": "%{BKY_MATH_HUE}",
     "tooltip": "%{BKY_MATH_IS_TOOLTIP}",
-    "extensions": ["math_is_divisibleby_mutator"]
+    "mutator": "math_is_divisibleby_mutator"
   },
 
   // Block for adding to a variable in place.
@@ -288,7 +288,8 @@ Blockly.defineBlocksWithJsonArray([  // BEGIN JSON EXTRACT
     "output": "Number",
     "colour": "%{BKY_MATH_HUE}",
     "helpUrl": "%{BKY_MATH_ONLIST_HELPURL}",
-    "extensions": ["math_op_tooltip", "math_modes_of_list_mutator"]
+    "mutator": "math_modes_of_list_mutator",
+    "extensions": ["math_op_tooltip"]
   },
 
   // Block for remainder of a division.
@@ -480,14 +481,14 @@ Blockly.Constants.Math.IS_DIVISIBLEBY_MUTATOR_MIXIN = {
  * @package
  */
 Blockly.Constants.Math.IS_DIVISIBLE_MUTATOR_EXTENSION = function() {
-  this.mixin(Blockly.Constants.Math.IS_DIVISIBLEBY_MUTATOR_MIXIN);
   this.getField('PROPERTY').setValidator(function(option) {
     var divisorInput = (option == 'DIVISIBLE_BY');
     this.sourceBlock_.updateShape_(divisorInput);
   });
 };
 
-Blockly.Extensions.register('math_is_divisibleby_mutator',
+Blockly.Extensions.registerMutator('math_is_divisibleby_mutator',
+  Blockly.Constants.Math.IS_DIVISIBLEBY_MUTATOR_MIXIN,
   Blockly.Constants.Math.IS_DIVISIBLE_MUTATOR_EXTENSION);
 
 /**
@@ -555,11 +556,11 @@ Blockly.Constants.Math.LIST_MODES_MUTATOR_MIXIN = {
  * @package
  */
 Blockly.Constants.Math.LIST_MODES_MUTATOR_EXTENSION = function() {
-  this.mixin(Blockly.Constants.Math.LIST_MODES_MUTATOR_MIXIN);
   this.getField('OP').setValidator(function(newOp) {
     this.updateType_(newOp);
   }.bind(this));
 };
 
-Blockly.Extensions.register('math_modes_of_list_mutator',
+Blockly.Extensions.registerMutator('math_modes_of_list_mutator',
+  Blockly.Constants.Math.LIST_MODES_MUTATOR_MIXIN,
   Blockly.Constants.Math.LIST_MODES_MUTATOR_EXTENSION);
diff --git a/core/extensions.js b/core/extensions.js
index 74833b5b2..e1d7ce782 100644
--- a/core/extensions.js
+++ b/core/extensions.js
@@ -92,11 +92,14 @@ Blockly.Extensions.registerMixin = function(name, mixinObj) {
  * decompose are defined on the mixin.
  * @param {string} name The name of this mutator extension.
  * @param {!Object} mixinObj The values to mix in.
- * @param {Array.<string>=} opt_blockList A list of blocks to appear in the flyout
- *     of the mutator dialog.
+ * @param {function()=} opt_helperFn An optional function to apply after mixing
+ *     in the object.
+ * @param {Array.<string>=} opt_blockList A list of blocks to appear in the
+ *     flyout of the mutator dialog.
  * @throws {Error} if the mutation is invalid or can't be applied to the block.
  */
-Blockly.Extensions.registerMutator = function(name, mixinObj, opt_blockList) {
+Blockly.Extensions.registerMutator = function(name, mixinObj, opt_helperFn,
+    opt_blockList) {
   var errorPrefix = 'Error when registering mutator "' + name + '": ';
 
   // Sanity check the mixin object before registering it.
@@ -106,13 +109,21 @@ Blockly.Extensions.registerMutator = function(name, mixinObj, opt_blockList) {
   var hasMutatorDialog = Blockly.Extensions.checkMutatorDialog(mixinObj,
     errorPrefix);
 
+  if (opt_helperFn && !goog.isFunction(opt_helperFn)) {
+    throw new Error('Extension "' + name + '" is not a function');
+  }
+
   // Sanity checks passed.
   Blockly.Extensions.register(name, function() {
     if (hasMutatorDialog) {
       this.setMutator(new Blockly.Mutator(opt_blockList));
     }
-    // Finally, mixin the object.
+    // Mixin the object.
     this.mixin(mixinObj);
+
+    if (opt_helperFn) {
+      opt_helperFn.apply(this);
+    }
   });
 };