Support user generated custom native JS classes.

pkg/compiler/lib/src/common/backend_api.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.
 
 library dart2js.backend_api;
 
 import 'dart:async' show Future;
 
 import '../compiler.dart' show
     Compiler;
@@ skipping 29 matching lines @@
 import '../io/source_information.dart' show
     SourceInformationStrategy;
 import '../js_backend/js_backend.dart' as js_backend show
     JavaScriptBackend;
 import '../library_loader.dart' show
     LibraryLoader,
     LoadedLibraries;
 import '../native/native.dart' as native show
     NativeEnqueuer;
 import '../patch_parser.dart' show
-    checkNativeAnnotation;
+    checkNativeAnnotation, checkJsInteropAnnotation;
 import '../resolution/tree_elements.dart' show
     TreeElements;
 import '../tree/tree.dart' show
     Node,
     Send;
 import '../universe/call_structure.dart' show
     CallStructure;
 
 import 'codegen.dart' show
     CodegenWorkItem;
@@ skipping 228 matching lines @@
   /// This method is called immediately after the [library] and its parts have
   /// been scanned.
   Future onLibraryScanned(LibraryElement library, LibraryLoader loader) {
     if (library.canUseNative) {
       library.forEachLocalMember((Element element) {
         if (element.isClass) {
           checkNativeAnnotation(compiler, element);
         }
       });
     }
+    checkJsInteropAnnotation(compiler, library);
+    library.forEachLocalMember((Element element) {
+      checkJsInteropAnnotation(compiler, element);
+      if (element.isClass) {

[sra1, 2015/10/06 21:42:17]

Is there a way to defer the member checking?
If the class has no annotation, are annotations on the members meaningful?

[Jacob, 2015/10/13 01:19:22]

They aren't. Added a short circuit based on 
element.isJsInterop

+        ClassElement classElement = element;
+        classElement.forEachMember((_, memberElement) {
+          checkJsInteropAnnotation(compiler, memberElement);
+        });
+      }
+    });
     return new Future.value();
   }
 
   /// This method is called when all new libraries loaded through
   /// [LibraryLoader.loadLibrary] has been loaded and their imports/exports
   /// have been computed.
   Future onLibrariesLoaded(LoadedLibraries loadedLibraries) {
     return new Future.value();
   }
 
@@ skipping 86 matching lines @@
   /// Returns the constant expression of [node], or `null` if [node] is not
   /// a constant expression.
   ConstantExpression getConstant(Node node);
 
   /// Registers [type] as instantiated.
   void registerInstantiatedType(InterfaceType type);
 
   /// Resolves [typeName] to a type in the context of [node].
   DartType resolveTypeFromString(Node node, String typeName);
 }