diff --git a/blocks/math.js b/blocks/math.js
index 9fbdde134..755d38dcb 100644
--- a/blocks/math.js
+++ b/blocks/math.js
@@ -467,7 +467,7 @@ Blockly.Extensions.register("math_is_divisibleby_mutator",
    * @this {Blockly.Block}
    */
   function() {
-    goog.mixin(this, Blockly.Blocks.math.IS_DIVISIBLEBY_MUTATOR_MIXIN_);
+    this.mixin(Blockly.Blocks.math.IS_DIVISIBLEBY_MUTATOR_MIXIN_);
     this.getField('PROPERTY').setValidator(function(option) {
       var divisorInput = (option == 'DIVISIBLE_BY');
       this.sourceBlock_.updateShape_(divisorInput);
@@ -526,9 +526,8 @@ Blockly.Extensions.register("math_modes_of_list_mutator",
    * @this {Blockly.Block}
    */
   function() {
-    var thisBlock = this;
-    goog.mixin(this, Blockly.Blocks.math.LIST_MODES_MUTATOR_MIXIN_);
+    this.mixin(Blockly.Blocks.math.LIST_MODES_MUTATOR_MIXIN_);
     this.getField('OP').setValidator(function(newOp) {
-      thisBlock.updateType_(newOp);
-    });
+      this.updateType_(newOp);
+    }.bind(this));
   });
diff --git a/core/block.js b/core/block.js
index 42cfaa74e..ff27cf152 100644
--- a/core/block.js
+++ b/core/block.js
@@ -1042,6 +1042,34 @@ Blockly.Block.prototype.jsonInit = function(json) {
   }
 };
 
+/**
+ * Add key/values from mixinObj to this block object. By default, this method
+ * will check that the keys in mixinObj will not overwrite existing values in
+ * the block, including prototype values. This provides some insurance against
+ * mixin / extension incompatibilities with future block features. This check
+ * can be disabled by passing true as the second argument.
+ * @param {!Object} mixinObj The key/values pairs to add to this block object.
+ * @param {boolean=} opt_disableCheck Option flag to disable overwrite checks.
+ */
+Blockly.Block.prototype.mixin = function(mixinObj, opt_disableCheck) {
+  if (goog.isDef(opt_disableCheck) && !goog.isBoolean(opt_disableCheck)) {
+    throw new Error("opt_disableCheck must be a boolean if provided");
+  }
+  if (!opt_disableCheck) {
+    var overwrites = [];
+    for (var key in mixinObj) {
+      if (this[key] !== undefined) {
+        overwrites.push(key);
+      }
+    }
+    if (overwrites.length) {
+      throw new Error('Mixin will overwrite block members: ' +
+        JSON.stringify(overwrites));
+    }
+  }
+  goog.mixin(this, mixinObj);
+};
+
 /**
  * Interpolate a message description onto the block.
  * @param {string} message Text contains interpolation tokens (%1, %2, ...)
diff --git a/core/extensions.js b/core/extensions.js
index 5259925bd..7ed26f3d7 100644
--- a/core/extensions.js
+++ b/core/extensions.js
@@ -63,6 +63,19 @@ Blockly.Extensions.register = function(name, initFn) {
   Blockly.Extensions.ALL_[name] = initFn;
 };
 
+/**
+ * Registers a new extension function that adds all key/value of mixinObj.
+ * @param {string} name The name of this extension.
+ * @param {!Object} mixinObj The values to mix in.
+ * @throws {Error} if the extension name is empty or the extension is already
+ *     registered.
+ */
+Blockly.Extensions.registerMixin = function(name, mixinObj) {
+  Blockly.Extensions.register(name, function() {
+    this.mixin(mixinObj);
+  });
+};
+
 /**
  * Applies an extension method to a block. This should only be called during
  * block construction.
diff --git a/tests/jsunit/extensions_test.js b/tests/jsunit/extensions_test.js
index caef7c992..5924f8e05 100644
--- a/tests/jsunit/extensions_test.js
+++ b/tests/jsunit/extensions_test.js
@@ -193,4 +193,114 @@ function test_parent_tooltip_when_inline() {
     delete Blockly.Blocks['test_parent_tooltip_when_inline'];
     delete Blockly.Blocks['test_parent'];
   }
-}
\ No newline at end of file
+}
+
+function test_mixin_extension() {
+  var TEST_MIXIN = {
+    field: 'FIELD',
+    method: function() {
+      console.log('TEXT_MIXIN method()');
+    }
+  };
+
+  var workspace = new Blockly.Workspace();
+  var block;
+  try {
+    assertUndefined(Blockly.Extensions.ALL_['mixin_test']);
+
+    // Extension defined before the block type is defined.
+    Blockly.Extensions.registerMixin('mixin_test', TEST_MIXIN);
+    assert(goog.isFunction(Blockly.Extensions.ALL_['mixin_test']));
+
+    Blockly.defineBlocksWithJsonArray([{
+      "type": "test_block_mixin",
+      "message0": "test_block_mixin",
+      "extensions": ["mixin_test"]
+    }]);
+
+    block = new Blockly.Block(workspace, 'test_block_mixin');
+
+    assertEquals(TEST_MIXIN.field, block.field);
+    assertEquals(TEST_MIXIN.method, block.method);
+  } finally {
+    block && block.dispose();
+    workspace.dispose();
+
+    delete Blockly.Extensions.ALL_['mixin_test'];
+    delete Blockly.Blocks['test_block_mixin'];
+  }
+}
+
+function test_bad_mixin_overwrites_local_value() {
+  var TEST_MIXIN_BAD_INPUTLIST = {
+    inputList: 'bad inputList'  // Defined in constructor
+  };
+
+  var workspace = new Blockly.Workspace();
+  var block;
+  try {
+    assertUndefined(Blockly.Extensions.ALL_['mixin_bad_inputList']);
+
+    // Extension defined before the block type is defined.
+    Blockly.Extensions.registerMixin('mixin_bad_inputList', TEST_MIXIN_BAD_INPUTLIST);
+    assert(goog.isFunction(Blockly.Extensions.ALL_['mixin_bad_inputList']));
+
+    Blockly.defineBlocksWithJsonArray([{
+      "type": "test_block_bad_inputList",
+      "message0": "test_block_bad_inputList",
+      "extensions": ["mixin_bad_inputList"]
+    }]);
+
+    try {
+      block = new Blockly.Block(workspace, 'test_block_bad_inputList');
+    } catch (e) {
+      // Expected Error
+      assert(e.message.indexOf('inputList') >= 0);  // Reference the conflict
+      return;
+    }
+    fail('Expected error when constructing block');
+  } finally {
+    block && block.dispose();
+    workspace.dispose();
+
+    delete Blockly.Extensions.ALL_['mixin_bad_inputList'];
+    delete Blockly.Blocks['test_block_bad_inputList'];
+  }
+}
+
+function test_bad_mixin_overwrites_prototype() {
+  var TEST_MIXIN_BAD_COLOUR = {
+    colour_: 'bad colour_' // Defined on prototype
+  };
+
+  var workspace = new Blockly.Workspace();
+  var block;
+  try {
+    assertUndefined(Blockly.Extensions.ALL_['mixin_bad_colour_']);
+
+    // Extension defined before the block type is defined.
+    Blockly.Extensions.registerMixin('mixin_bad_colour_', TEST_MIXIN_BAD_COLOUR);
+    assert(goog.isFunction(Blockly.Extensions.ALL_['mixin_bad_colour_']));
+
+    Blockly.defineBlocksWithJsonArray([{
+      "type": "test_block_bad_colour",
+      "message0": "test_block_bad_colour",
+      "extensions": ["mixin_bad_colour_"]
+    }]);
+
+    try {
+      block = new Blockly.Block(workspace, 'test_block_bad_colour');
+    } catch (e) {
+      // Expected Error
+      assert(e.message.indexOf('colour_') >= 0);  // Reference the conflict
+      return;
+    }
+    fail('Expected error when constructing block');
+  } finally {
+    block && block.dispose();
+    workspace.dispose();
+
+    delete Blockly.Extensions.ALL_['mixin_bad_colour_'];
+    delete Blockly.Blocks['test_block_bad_colour'];
+  }
+}