Support is!.

dart/frog/leg/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);
 }
 
 class TreeElementMapping implements TreeElements {
@@ skipping 562 matching lines @@
         target = prefix.library.lookupLocalMember(name);
         if (target == null) {
           error(node, MessageKind.NO_SUCH_LIBRARY_MEMBER,
                 [resolvedReceiver.name, name]);
         }
       }
     }
     return target;
   }
 
-  resolveTypeTest(TypeAnnotation node) {
+  resolveTypeTest(Node argument) {
+    TypeAnnotation node = argument.asTypeAnnotation();
+    if (node == null) {
+      node = argument.asSend().receiver;
+    }
     resolveTypeRequired(node);
   }
 
   void handleArguments(Send node) {
     int count = 0;
     List<SourceString> namedArguments = <SourceString>[];
     for (Link<Node> link = node.argumentsNode.nodes;
          !link.isEmpty();
          link = link.tail) {
       count++;
@@ skipping 556 matching lines @@
 class TopScope extends Scope {
   LibraryElement get library() => element;
 
   TopScope(LibraryElement library) : super(null, library);
   Element lookup(SourceString name) => library.find(name);
 
   Element add(Element element) {
     throw "Cannot add an element in the top scope";
   }
 }