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

pkg/compiler/lib/src/js_emitter/native_emitter.dart

Index: pkg/compiler/lib/src/js_emitter/native_emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/native_emitter.dart b/pkg/compiler/lib/src/js_emitter/native_emitter.dart
index 6423959d2c125f3bbee637edfa9c55d071d5fb7e..843fd8750e14524cceab1f72a82b54d95b1b73f6 100644
--- a/pkg/compiler/lib/src/js_emitter/native_emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/native_emitter.dart
@@ -125,6 +125,7 @@ class NativeEmitter {
       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.
@@ -230,7 +231,9 @@ class NativeEmitter {
       }
       generateClassInfo(jsInterceptorClass);
       for (Class cls in classes) {
-        generateClassInfo(cls);
+        if (!cls.isNative || neededClasses.contains(cls)) {
+          generateClassInfo(cls);
+        }
       }
     }