Fix type annotations that violate subtyping constraint on overriden methods.

pkg/compiler/lib/src/resolution/signatures.dart

 // Copyright (c) 2014, 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 resolution;
 
 /**
  * [SignatureResolver] resolves function signatures.
  */
 class SignatureResolver extends MappingVisitor<FormalElementX> {
@@ skipping 108 matching lines @@
             link.head.asIdentifier() != null || link.head.asSend() != null));
         if (fieldElement != null) {
           element.typeCache = fieldElement.computeType(compiler);
         } else {
           element.typeCache = const DynamicType();
         }
       }
     }
   }
 
-  Element visitIdentifier(Identifier node) {
+  FormalElementX visitIdentifier(Identifier node) {
     return createParameter(node, null);
   }
 
   Identifier getParameterName(Send node) {
     var identifier = node.selector.asIdentifier();
     if (identifier != null) {
       // Normal parameter: [:Type name:].
       return identifier;
     } else {
       // Function type parameter: [:void name(DartType arg):].
@@ skipping 240 matching lines @@
 
   DartType resolveReturnType(TypeAnnotation annotation) {
     if (annotation == null) return const DynamicType();
     DartType result = resolver.resolveTypeAnnotation(annotation);
     if (result == null) {
       return const DynamicType();
     }
     return result;
   }
 }