Fix Array constructor bug impacting dart2js and ddc running on the same page.

lib/runtime/dart_runtime.js

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 var dart, _js_helper;
 (function (dart) {
   'use strict';
 
   let defineProperty = Object.defineProperty;
   let getOwnPropertyDescriptor = Object.getOwnPropertyDescriptor;
@@ skipping 593 matching lines @@
   function defineLazy(to, from) {
     for (let name of getOwnNamesAndSymbols(from)) {
       defineLazyProperty(to, name, getOwnPropertyDescriptor(from, name));
     }
   }
   // TODO(jmesserly): these are identical, but this makes it easier to grep for.
   dart.defineLazyClass = defineLazy;
   dart.defineLazyProperties = defineLazy;
   dart.defineLazyClassGeneric = defineLazyProperty;
 
+  function copyPropertiesHelper(to, from, names) {
+    for (let name of names) {
+      defineProperty(to, name, getOwnPropertyDescriptor(from, name));
+    }
+    return to;
+  }
+
   /**
    * Copy properties from source to destination object.
    * This operation is commonly called `mixin` in JS.
    */
   function copyProperties(to, from) {
-    for (let name of getOwnNamesAndSymbols(from)) {
-      defineProperty(to, name, getOwnPropertyDescriptor(from, name));
-    }
-    return to;
+    return copyPropertiesHelper(to, from, getOwnNamesAndSymbols(from));
   }
+
+  /**
+   * Copy symbols from the prototype of the source to destination.
+   * These are the only properties safe to copy onto an existing public
+   * JavaScript class.
+   */
+  function registerExtension(to, from) {
+    return copyPropertiesHelper(to.prototype, from.prototype,
+        getOwnPropertySymbols(from.prototype));
+  }
+
   dart.copyProperties = copyProperties;
+  dart.registerExtension = registerExtension;
 
   /**
    * This is called whenever a derived class needs to introduce a new field,
    * shadowing a field or getter/setter pair on its parent.
    *
    * This is important because otherwise, trying to read or write the field
    * would end up calling the getter or setter, and one of those might not even
    * exist, resulting in a runtime error. Even if they did exist, that's the
    * wrong behavior if a new field was declared.
    */
@@ skipping 223 matching lines @@
   dart.bottom = { toString() { return 'bottom'; } };
 
   dart.global = window || global;
   dart.JsSymbol = Symbol;
 
   // TODO(jmesserly): hack to bootstrap the SDK
   _js_helper = _js_helper || {};
   _js_helper.checkNum = notNull;
 
 })(dart || (dart = {}));