Support user generated custom native JS classes.

pkg/compiler/lib/src/js_emitter/native_emitter.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 dart2js.js_emitter;
 
 class NativeEmitter {
 
   // TODO(floitsch): the native-emitter should not know about ClassBuilders.
   final Map<Element, full_js_emitter.ClassBuilder> cachedBuilders;
@@ skipping 116 matching lines @@
         needed = true;
       } else if (interceptorClassesNeededByConstants.contains(classElement)) {
         needed = true;
       } else if (classesModifiedByEmitRTISupport.contains(classElement)) {
         // TODO(9556): Remove this test when [emitRuntimeTypeSupport] no longer
         // adds information to a class prototype or constructor.
         needed = true;
       } else if (extensionPoints.containsKey(cls)) {
         needed = true;
       }
-      if (cls.isNative &&
-          native.nativeTagsForcedNonLeaf(classElement)) {
+      if (classElement.isJsInterop) {
+        needed = false;
+      } else if (cls.isNative && native.nativeTagsForcedNonLeaf(classElement)) {
         needed = true;
         nonLeafClasses.add(cls);
       }
 
       if (needed || neededClasses.contains(cls)) {
         neededClasses.add(cls);
         neededClasses.add(cls.superclass);
         nonLeafClasses.add(cls.superclass);
       }
     }
 
     // Collect all the tags that map to each native class.
 
     Map<Class, Set<String>> leafTags = new Map<Class, Set<String>>();
     Map<Class, Set<String>> nonleafTags = new Map<Class, Set<String>>();
 
     for (Class cls in classes) {
       if (!cls.isNative) continue;
+      if (cls.element.isJsInterop) continue;
       List<String> nativeTags = native.nativeTagsOfClass(cls.element);
 
       if (nonLeafClasses.contains(cls) ||
           extensionPoints.containsKey(cls)) {
         nonleafTags
             .putIfAbsent(cls, () => new Set<String>())
             .addAll(nativeTags);
       } else {
         Class sufficingInterceptor = cls;
         while (!neededClasses.contains(sufficingInterceptor)) {
@@ skipping 176 matching lines @@
     // used.  We should also use an interceptor if the check can't be satisfied
     // by a native class in case we get a native instance that tries to spoof
     // the type info.  i.e the criteria for whether or not to use an interceptor
     // is whether the receiver can be native, not the type of the test.
     if (element == null || !element.isClass) return false;
     ClassElement cls = element;
     if (Elements.isNativeOrExtendsNative(cls)) return true;
     return isSupertypeOfNativeClass(element);
   }
 }