Improve checking of patches.

sdk/lib/_internal/compiler/implementation/resolution/members.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.
 
 part of resolution;
 
 abstract class TreeElements {
   Element operator[](Node node);
   Selector getSelector(Send send);
   DartType getType(Node node);
@@ skipping 232 matching lines @@
         assert(isConstructor);
         return elements;
       }
       if (element.isPatched) {
         checkMatchingPatchSignatures(element, element.patch);
         element = element.patch;
       }
       return compiler.withCurrentElement(element, () {
         FunctionExpression tree = element.parseNode(compiler);
         if (tree.modifiers.isExternal()) {
-          error(tree, MessageKind.EXTERNAL_WITHOUT_IMPLEMENTATION);
+          error(tree, MessageKind.PATCH_EXTERNAL_WITHOUT_IMPLEMENTATION);
           return;
         }
         if (isConstructor) {
           if (tree.returnType != null) {
             error(tree, MessageKind.CONSTRUCTOR_WITH_RETURN_TYPE);
           }
           resolveConstructorImplementation(element, tree);
         }
         ResolverVisitor visitor = visitorFor(element);
         visitor.useElement(tree, element);
@@ skipping 3385 matching lines @@
     return e;
   }
 
   /// Assumed to be called by [resolveRedirectingFactory].
   Element visitReturn(Return node) {
     Node expression = node.expression;
     return finishConstructorReference(visit(expression),
                                       expression, expression);
   }
 }