Support parameter metadata.

sdk/lib/_internal/compiler/implementation/scanner/class_element_parser.dart

 // Copyright (c) 2011, 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 scanner;
 
 class ClassElementParser extends PartialParser {
   ClassElementParser(Listener listener) : super(listener);
 
   Token parseClassBody(Token token) => fullParseClassBody(token);
@@ skipping 25 matching lines @@
 
   ClassNode parseNode(Compiler compiler) {
     if (cachedNode != null) return cachedNode;
     compiler.withCurrentElement(this, () {
       compiler.parser.measure(() {
         MemberListener listener = new MemberListener(compiler, this);
         Parser parser = new ClassElementParser(listener);
         Token token = parser.parseTopLevelDeclaration(beginToken);
         assert(identical(token, endToken.next));
         cachedNode = listener.popNode();
-        assert(listener.nodes.isEmpty);
+        assert(invariant(beginToken, listener.nodes.isEmpty,
+            message: "Non-empty listener stack: ${listener.nodes}"));
       });
       compiler.patchParser.measure(() {
         if (isPatched) {
           // TODO(lrn): Perhaps extract functionality so it doesn't
           // need compiler.
           compiler.patchParser.parsePatchClassNode(patch);
         }
       });
     });
     return cachedNode;
@@ skipping 136 matching lines @@
 
   void endMetadata(Token beginToken, Token periodBeforeName, Token endToken) {
     popNode(); // Discard arguments.
     if (periodBeforeName != null) {
       popNode(); // Discard name.
     }
     popNode(); // Discard node (Send or Identifier).
     pushMetadata(new PartialMetadataAnnotation(beginToken, endToken));
   }
 }