Report compile time error if named argument is given twice.

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 1753 matching lines @@
 
   Selector resolveSelector(Send node) {
     LibraryElement library = enclosingElement.getLibrary();
     Selector selector = computeSendSelector(node, library);
     if (selector != null) mapping.setSelector(node, selector);
     return selector;
   }
 
   void resolveArguments(NodeList list) {
     if (list == null) return;
-    bool seenNamedArgument = false;
+    List<SourceString> seenNamedArguments = <SourceString>[];
     for (Link<Node> link = list.nodes; !link.isEmpty; link = link.tail) {
       Expression argument = link.head;
       visit(argument);
-      if (argument.asNamedArgument() != null) {
-        seenNamedArgument = true;
-      } else if (seenNamedArgument) {
+      NamedArgument namedArgument = argument.asNamedArgument();
+      if (namedArgument != null) {
+        SourceString source = namedArgument.name.source;
+        if (seenNamedArguments.contains(source)) {
+          error(argument, MessageKind.DUPLICATE_DEFINITION,
+                [source.slowToString()]);
+        }
+        seenNamedArguments.add(source);
+      } else if (!seenNamedArguments.isEmpty) {
         error(argument, MessageKind.INVALID_ARGUMENT_AFTER_NAMED);
       }
     }
   }
 
   visitSend(Send node) {
     Element target = resolveSend(node);
     if (!Elements.isUnresolved(target)
         && target.kind == ElementKind.ABSTRACT_FIELD) {
       AbstractFieldElement field = target;
@@ skipping 1419 matching lines @@
     return e;
   }
 
   /// Assumed to be called by [resolveRedirectingFactory].
   Element visitReturn(Return node) {
     Node expression = node.expression;
     return finishConstructorReference(visit(expression),
                                       expression, expression);
   }
 }