Add JSNumber, JSInt and JSDouble for the new interceptor scheme.

sdk/lib/_internal/compiler/implementation/lib/js_string.dart

Index: sdk/lib/_internal/compiler/implementation/lib/js_string.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/lib/js_string.dart	(revision 15078)
+++ sdk/lib/_internal/compiler/implementation/lib/js_string.dart	(working copy)
@@ -123,5 +123,11 @@
     return stringContainsUnchecked(this, other, startIndex);
   }
 
-  bool get isEmpty => length == 0;  
+  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;
+  }
 }