Make sure we follow the spec's native prototype resolution for IE10 p…

…roto-mixin

Fixes #108

add test for proper prototype

src/CustomElements.js

@@ -201,7 +201,11 @@ if (useNative) {
       // work out prototype when using type-extension
       if (definition.is) {
         var inst = document.createElement(definition.tag);
-        nativePrototype = Object.getPrototypeOf(inst);
+        var expectedPrototype = Object.getPrototypeOf(inst);
+        // only set nativePrototype if it will actually appear in the definition's chain
+        if (expectedPrototype === definition.prototype) {
+          nativePrototype = expectedPrototype;
+        }
       }
       // ensure __proto__ reference is installed at each point on the prototype
       // chain.
@@ -210,13 +214,13 @@ if (useNative) {
       // limited support for prototype traversal.
       var proto = definition.prototype, ancestor;
       while (proto && (proto !== nativePrototype)) {
-        var ancestor = Object.getPrototypeOf(proto);
+        ancestor = Object.getPrototypeOf(proto);
         proto.__proto__ = ancestor;
         proto = ancestor;
       }
+      // cache this in case of mixin
+      definition.native = nativePrototype;
     }
-    // cache this in case of mixin
-    definition.native = nativePrototype;
   }
 
   // SECTION 4

test/js/customElements.js

@@ -481,7 +481,19 @@
     assert.isTrue(CustomElements.instanceof(x2, PCtor), 'instanceof failed for x-button-instance2');
     assert.isTrue(CustomElements.instanceof(x2, HTMLButtonElement), 'instanceof failed for x-button-instance2');
   });
-
+
+  test('extends and prototype mismatch', function() {
+    var p = Object.create(HTMLElement.prototype);
+    var PCtor = document.registerElement('not-button', {
+      extends: 'button',
+      prototype: p
+    });
+
+    var e = document.createElement('button', 'not-button');
+    assert.isFalse(CustomElements.instanceof(e, HTMLButtonElement));
+    assert.isTrue(CustomElements.instanceof(e, HTMLElement));
+  });
+
 });
 
 htmlSuite('customElements (html)', function() {