use == and != to enable handling null/undefined

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';
 
   var defineProperty = Object.defineProperty;
   var getOwnPropertyDescriptor = Object.getOwnPropertyDescriptor;
@@ skipping 110 matching lines @@
   }
   dart.isGroundType = isGroundType;
 
   function arity(f) {
     // TODO(vsm): Implement.
     throw new core.UnimplementedError();
   }
   dart.arity = arity;
 
   function equals(x, y) {
-    if (x === null || y === null) return x === y;
+    if (x == null || y == null) return x == y;
     var eq = x['=='];
-    return eq ? eq.call(x, y) : x === y;
+    return eq ? eq.call(x, y) : x == y;
   }
   dart.equals = equals;
 
   /** Checks that `x` is not null or undefined. */
   function notNull(x) {
     if (x == null) throw 'expected not-null value';
     return x;
   }
   dart.notNull = notNull;
 
@@ skipping 195 matching lines @@
   // of some sort, assuming we keep around `dynamic` at runtime.
   dart.dynamic = Object.create(null);
 
   dart.JsSymbol = Symbol;
 
   // TODO(jmesserly): hack to bootstrap the SDK
   _js_helper = _js_helper || {};
   _js_helper.checkNum = notNull;
 
 })(dart || (dart = {}));