Support is!.

dart/frog/leg/ssa/builder.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 Interceptors {
   Compiler compiler;
   Interceptors(Compiler this.compiler);
 
   SourceString mapOperatorToMethodName(Operator op) {
     String name = op.source.stringValue;
@@ skipping 1202 matching lines @@
     } else if (const SourceString("&&") == op.source ||
                const SourceString("||") == op.source) {
       visitLogicalAndOr(node, op);
     } else if (const SourceString("!") == op.source) {
       visitLogicalNot(node);
     } else if (node.argumentsNode is Prefix) {
       visitUnary(node, op);
     } else if (const SourceString("is") == op.source) {
       visit(node.receiver);
       HInstruction expression = pop();
-      push(new HIs(elements[node.arguments.head], expression));
+      Node argument = node.arguments.head;
+      TypeAnnotation type = argument.asTypeAnnotation();
+      bool isNot = false;
+      if (type == null) {
+        type = argument.asSend().receiver;

[ahe, 2012/02/13 09:59:57]

You're repeating the pattern from the resolver here.

+        isNot = true;
+      }
+      HInstruction instruction = new HIs(elements[type], expression);
+      if (isNot) {
+        add(instruction);
+        instruction = new HNot(instruction);
+      }
+      push(instruction);
     } else {
       visit(node.receiver);
       visit(node.argumentsNode);
       var right = pop();
       var left = pop();
       visitBinary(left, op, right);
     }
   }
 
   void addDynamicSendArgumentsToList(Send node, List<HInstruction> list) {
@@ skipping 709 matching lines @@
   }
 
   visitCatchBlock(CatchBlock node) {
     visit(node.block);
   }
 
   visitTypedef(Typedef node) {
     compiler.unimplemented('SsaBuilder.visitTypedef', node: node);
   }
 }