Support for generic type variables and unresolved types added.

lib/compiler/implementation/dart_backend/placeholder.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.
 
 class Placeholder {
+  const Placeholder();
   abstract String rename(ConflictingRenamer renamer);
 }
 
 class NullPlaceholder extends Placeholder {
   String rename(ConflictingRenamer renamer) => '';
   String toString() => 'null_placeholder[]';
 }
 
 class PrivatePlaceholder extends Placeholder {
   final LibraryElement library;
   final Identifier node;
   PrivatePlaceholder(this.library, this.node);
   String rename(ConflictingRenamer renamer) =>
       renamer.renamePrivateIdentifier(library, node.source.slowToString());
   String toString() => 'private_placeholder[node($node), $library]';
 }
 
 class ElementPlaceholder extends Placeholder {
   final Element element;
   ElementPlaceholder(this.element);
   String rename(ConflictingRenamer renamer) => renamer.renameElement(element);
   String toString() => 'element_placeholder[$element]';
 }
+
+class UnresolvedPlaceholder extends Placeholder {
+  const UnresolvedPlaceholder();
+  String rename(ConflictingRenamer renamer) =>
+      renamer.generateUniqueName('unresolved');
+  String toString() => 'unresolved_placeholder';
+}