dart2js: don't emit unneeded native info.

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

Index: pkg/compiler/lib/src/js_emitter/program_builder.dart
diff --git a/pkg/compiler/lib/src/js_emitter/program_builder.dart b/pkg/compiler/lib/src/js_emitter/program_builder.dart
index 2d22ead02315ee95bb458efb7d9fb5dbc76f3817..69f2fc603232661a8fd7f279ca031a8a4237b4df 100644
--- a/pkg/compiler/lib/src/js_emitter/program_builder.dart
+++ b/pkg/compiler/lib/src/js_emitter/program_builder.dart
@@ -117,18 +117,20 @@ class ProgramBuilder {
 
     _markEagerClasses();
 
-    bool containsNativeClasses =
-        nativeClasses.length != _unneededNativeClasses.length;
-
     List<Holder> holders = _registry.holders.toList(growable: false);
 
+    bool needsNativeSupport = _compiler.enqueuer.codegen.nativeEnqueuer
+        .hasInstantiatedNativeClasses();
+
+    assert(!needsNativeSupport || nativeClasses.isNotEmpty);
+
     return new Program(
         fragments,
         holders,
         _buildLoadMap(),
         _buildTypeToInterceptorMap(),
         _task.metadataCollector,
-        outputContainsNativeClasses: containsNativeClasses,
+        needsNativeSupport: needsNativeSupport,
         outputContainsConstantList: _task.outputContainsConstantList,
         hasIsolateSupport: _compiler.hasIsolateSupport);
   }