Fix inconsistency with null vs undefined in dart:js

sdk/lib/js/dart2js/js_dart2js.dart

 // Copyright (c) 2013, 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.
 
 /**
  * Support for interoperating with JavaScript.
  *
  * This library provides access to JavaScript objects from Dart, allowing
  * Dart code to get and set properties, and call methods of JavaScript objects
  * and invoke JavaScript functions. The library takes care of converting
@@ skipping 477 matching lines @@
   }
   return null;
 }
 
 bool _isLocalObject(o) => JS('bool', '# instanceof Object', o);
 
 // The shared constructor function for proxies to Dart objects in JavaScript.
 final _dartProxyCtor = JS('', 'function DartObject(o) { this.o = o; }');
 
 dynamic _convertToJS(dynamic o) {
-  if (o == null) {
-    return null;
-  } else if (o is String || o is num || o is bool) {
+  // Note: we don't write `if (o == null) return null;` to make sure dart2js
+  // doesn't convert `return null;` into `return;` (which would make `null` be
+  // `undefined` in Javascprit). See dartbug.com/20305 for details.
+  if (o == null || o is String || o is num || o is bool) {
     return o;
   } else if (o is Blob || o is Event || o is KeyRange || o is ImageData
       || o is Node || o is TypedData || o is Window) {
     return o;
   } else if (o is DateTime) {
     return Primitives.lazyAsJsDate(o);
   } else if (o is JsObject) {
     return o._jsObject;
   } else if (o is Function) {
     return _getJsProxy(o, _JS_FUNCTION_PROPERTY_NAME, (o) {
@@ skipping 62 matching lines @@
   // Dart in that context. The JS object will have a cached proxy
   // but it won't be a valid Dart object in this context.
   // For now we throw away the cached proxy, but we should be able
   // to cache proxies from multiple JS contexts and Dart isolates.
   if (dartProxy == null || !_isLocalObject(o)) {
     dartProxy = createProxy(o);
     _defineProperty(o, propertyName, dartProxy);
   }
   return dartProxy;
 }