fixes #40, extension methods for primitive types

test/generated_sdk/lib/_internal/compiler/js_lib/js_string.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.
 
 part of _interceptors;
 
 /**
  * The interceptor class for [String]. The compiler recognizes this
  * class as an interceptor, and changes references to [:this:] to
  * actually use the receiver of the method, which is generated as an extra
  * argument added to each member.
  */
+@JsPeerInterface(name: 'String')
 class JSString extends Interceptor implements String, JSIndexable {
   const JSString();
 
   int codeUnitAt(int index) {
     if (index is !int) throw new ArgumentError(index);
     if (index < 0) throw new RangeError.value(index);
     if (index >= length) throw new RangeError.value(index);
     return JS('JSUInt31', r'#.charCodeAt(#)', this, index);
   }
 
@@ skipping 439 matching lines @@
  */
 class _CodeUnits extends UnmodifiableListBase<int> {
   /** The string that this is the code units of. */
   String _string;
 
   _CodeUnits(this._string);
 
   int get length => _string.length;
   int operator[](int i) => _string.codeUnitAt(i);
 }