Add JSNull, JSBool and JSFunction, and move toString into the new interceptor scheme.

sdk/lib/_internal/compiler/implementation/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
@@ skipping 112 matching lines @@
     return stringContainsUnchecked(this, other, startIndex);
   }
 
   bool get isEmpty => length == 0;
 
   int compareTo(String other) {
     if (other is !String) throw new ArgumentError(other);
     return this == other ? 0
       : JS('bool', r'# < #', this, other) ? -1 : 1;
   }
+
+  String toString() => this;
 }