Fix metadata processing

pkg/compiler/lib/src/js_backend/backend.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.
 
 library js_backend.backend;
 
 import 'dart:async' show Future;
 
 import 'package:js_runtime/shared/embedded_names.dart' as embeddedNames;
 
@@ skipping 2343 matching lines @@
 
     void processElementMetadata(Element element) {
       if (_registeredMetadata.add(element)) {
         element.metadata.forEach(onMetadata);
         if (element.isFunction) {
           FunctionElement function = element;
           for (ParameterElement parameter in function.parameters) {
             parameter.metadata.forEach(onMetadata);
           }
         }
-        if (element.enclosingElement is ClassElement) {
-          // Use [enclosingElement] instead of [enclosingClass] to ensure that
-          // we process patch class metadata for patch and injected members.
-          processElementMetadata(element.enclosingElement);
+        if (element.enclosingClass != null) {
+          // Only process library of top level fields/methods
+          // (and not for classes).
+          // TODO(johnniwinther): Fix this: We are missing some metadata on
+          // libraries (example: in co19/Language/Metadata/before_export_t01).
+          if (element.enclosingElement is ClassElement) {
+            // Use [enclosingElement] instead of [enclosingClass] to ensure that
+            // we process patch class metadata for patch and injected members.
+            processElementMetadata(element.enclosingElement);
+          }
         } else {
           processLibraryMetadata(element.library);
         }
       }
     }
 
     entities.forEach(processElementMetadata);
   }
 
   void onQueueClosed() {
@@ skipping 899 matching lines @@
   @override
   bool isNativeClass(ClassElement element) {
     return helpers.backend.isNative(element);
   }
 
   @override
   bool isNativeMember(MemberElement element) {
     return helpers.backend.isNative(element);
   }
 }