Implement 'as' operator.

lib/compiler/implementation/resolver.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.
 
 interface TreeElements {
   Element operator[](Node node);
   Selector getSelector(Send send);
   Type getType(TypeAnnotation annotation);
 }
 
@@ skipping 1010 matching lines @@
         error(argument, MessageKind.INVALID_ARGUMENT_AFTER_NAMED);
       }
     }
     mapping.setSelector(node, new Selector.invocation(count, namedArguments));
   }
 
   visitSend(Send node) {
     Element target = resolveSend(node);
     if (node.isOperator) {
       Operator op = node.selector.asOperator();
-      if (op.source.stringValue === 'is') {
+      if (op.source.stringValue === 'is' || op.source.stringValue === 'as') {
         resolveTypeTest(node.arguments.head);
         assert(node.arguments.tail.isEmpty());
         mapping.setSelector(node, Selector.BINARY_OPERATOR);
       } else if (node.arguments.isEmpty()) {
         assert(op.token.kind !== PLUS_TOKEN);
         mapping.setSelector(node, Selector.UNARY_OPERATOR);
       } else {
         visit(node.argumentsNode);
         mapping.setSelector(node, Selector.BINARY_OPERATOR);
       }
@@ skipping 962 matching lines @@
 
   TopScope(LibraryElement library) : super(null, library);
   Element lookup(SourceString name) {
     return library.find(name);
   }
 
   Element add(Element newElement) {
     throw "Cannot add an element in the top scope";
   }
 }