Add experimental expando support.

lib/compiler/implementation/lib/js_helper.dart

 // Copyright (c) 2012, 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.
 
 #library('js_helper');
 
 #import('coreimpl.dart');
 
 #source('constant_map.dart');
 #source('native_helper.dart');
@@ skipping 549 matching lines @@
       : JS('int', @'#.getDay()', lazyAsJsDate(receiver));
   }
 
   static valueFromDateString(str) {
     checkNull(str);
     if (str is !String) throw new IllegalArgumentException(str);
     var value = JS('num', @'Date.parse(#)', str);
     if (value.isNaN()) throw new IllegalArgumentException(str);
     return value;
   }
+
+  static getProperty(object, key) {
+    checkNull(object);
+    if (object is bool || object is num || object is String) {
+      throw new IllegalArgumentException(object);
+    }
+    return JS('var', '#[#]', object, key);
+  }
+
+  static void setProperty(object, key, value) {
+    checkNull(object);
+    if (object is bool || object is num || object is String) {
+      throw new IllegalArgumentException(object);
+    }
+    JS('void', '#[#] = #', object, key, value);
+  }
 }
 
 /**
  * Called by generated code to throw an illegal-argument exception,
  * for example, if a non-integer index is given to an optimized
  * indexed access.
  */
 iae(argument) {
   throw new IllegalArgumentException(argument);
 }
@@ skipping 485 matching lines @@
   if (JS('bool', '!!#[#]', value, property)) return value;
   propertyTypeError(value, property);
 }
 
 listSuperNativeTypeCheck(value, property) {
   if (value === null) return value;
   if (value is List) return value;
   if (JS('bool', '#[#]()', value, property)) return value;
   propertyTypeError(value, property);
 }