Create an outline of metadata API in dart2js.

dart/lib/compiler/implementation/compiler.dart

Index: dart/lib/compiler/implementation/compiler.dart
diff --git a/dart/lib/compiler/implementation/compiler.dart b/dart/lib/compiler/implementation/compiler.dart
index e17e661538d1e27f260fd11c35ca130b636c82b6..dc9e08e0a4acb4d9b99b1bc54e852c1167c4694b 100644
--- a/dart/lib/compiler/implementation/compiler.dart
+++ b/dart/lib/compiler/implementation/compiler.dart
@@ -440,7 +440,10 @@ class Compiler implements DiagnosticListener {
   bool isPatchElement(Element element) {
     // TODO(lrn): More checks needed if we introduce metadata for real.
     // In that case, it must have the identifier "native" as metadata.
-    return !element.metadata.isEmpty();
+    for (Link link = element.metadata; !link.isEmpty(); link = link.tail) {
+      if (link.head is PatchMetadataAnnotation) return true;
+    }
+    return false;
   }
 
   Element clonePatch(Element patchElement, Element enclosing) {