Support user generated custom native JS classes.

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

 // Copyright (c) 2014, 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 dart2js.js_backend.patch_resolver;
 
 import '../common/tasks.dart' show
     CompilerTask;
 import '../compiler.dart' show
     Compiler;
@@ skipping 12 matching lines @@
 
   FunctionElement resolveExternalFunction(FunctionElementX element) {
     if (element.isPatched) {
       FunctionElementX patch = element.patch;
       compiler.withCurrentElement(patch, () {
         patch.parseNode(compiler);
         patch.computeType(compiler);
       });
       checkMatchingPatchSignatures(element, patch);
       element = patch;
-    } else {
-      compiler.reportError(
-         element, MessageKind.PATCH_EXTERNAL_WITHOUT_IMPLEMENTATION);
     }
     return element;
   }
 
   void checkMatchingPatchParameters(FunctionElement origin,
                                     List<Element> originParameters,
                                     List<Element> patchParameters) {
 
     assert(originParameters.length == patchParameters.length);
     for (int index = 0; index < originParameters.length; index++) {
@@ skipping 105 matching lines @@
              'patchParameterCount': patchSignature.optionalParameterCount});
       });
     } else {
       checkMatchingPatchParameters(origin,
                                    originSignature.optionalParameters,
                                    patchSignature.optionalParameters);
     }
   }
 
 }