dart2js: Don't add properties to native classes that are not needed.

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 {
 
   final Map<Element, ClassBuilder> cachedBuilders;
 
@@ skipping 107 matching lines @@
         .computeClassesModifiedByEmitRuntimeTypeSupport();
 
     for (Class cls in preOrder.reversed) {
       ClassElement classElement = cls.element;
       // Post-order traversal ensures we visit the subclasses before their
       // superclass.  This makes it easy to tell if a class is needed because a
       // subclass is needed.
       ClassBuilder builder = builders[cls];
       bool needed = false;
       if (builder == null) {
+        assert(!cls.isNative);
         // Mixin applications (native+mixin) are non-native, so [classElement]
         // has already been emitted as a regular class.  Mark [classElement] as
         // 'needed' to ensure the native superclass is needed.
         needed = true;
       } else if (!builder.isTrivial) {
         needed = true;
       } else if (neededByConstant.contains(classElement)) {
         needed = true;
       } else if (modifiedClasses.contains(classElement)) {
         // TODO(9556): Remove this test when [emitRuntimeTypeSupport] no longer
@@ skipping 85 matching lines @@
                     () => new Map<String, jsAst.Expression>());
             properties[backend.namer.nativeSpecProperty] = js.string(encoding);
           }
         } else {
           builder.addProperty(
               backend.namer.nativeSpecProperty, js.string(encoding));
         }
       }
       generateClassInfo(jsInterceptorClass);
       for (Class cls in classes) {
-        generateClassInfo(cls);
+        if (!cls.isNative || neededClasses.contains(cls)) {
+          generateClassInfo(cls);
+        }
       }
     }
 
     // Emit the native class interceptors that were actually used.
     for (Class cls in classes) {
       ClassElement classElement = cls.element;
       if (!cls.isNative) continue;
       if (neededClasses.contains(cls)) {
         ClassBuilder builder = builders[classElement];
 
@@ skipping 242 matching lines @@
         targetOutput.add(';');
       }
       targetOutput.addBuffer(jsAst.prettyPrint(
           new jsAst.ExpressionStatement(init), compiler));
       targetOutput.add('\n');
     }
 
     targetOutput.add('\n');
   }
 }