Support user generated custom native JS classes.

pkg/compiler/lib/src/js_backend/namer.dart

 // Copyright (c) 2011, 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 js_backend;
 
 /**
  * Assigns JavaScript identifiers to Dart variables, class-names and members.
  *
  * Names are generated through three stages:
@@ skipping 746 matching lines @@
 
   /**
    * Returns the JavaScript property name used to store an instance field.
    */
   jsAst.Name instanceFieldPropertyName(FieldElement element) {
     ClassElement enclosingClass = element.enclosingClass;
 
     if (element.hasFixedBackendName) {
       // Certain native fields must be given a specific name. Native names must
       // not contain '$'. We rely on this to avoid clashes.
+      // TODO(jacobr): we need to relax this constraint.
       assert(enclosingClass.isNative &&
              !element.fixedBackendName.contains(r'$'));
 
       return new StringBackedName(element.fixedBackendName);
     }
 
     // Instances of BoxFieldElement are special. They are already created with
     // a unique and safe name. However, as boxes are not really instances of
     // classes, the usual naming scheme that tries to avoid name clashes with
     // super classes does not apply. We still do not mark the name as a
@@ skipping 1205 matching lines @@
       }
     }
   }
 }
 
 enum NamingScope {
   global,
   instance,
   constant
 }