Support for loops in analyzer2dart.

sdk/lib/_internal/compiler/implementation/tree/nodes.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 tree;
 
 abstract class Visitor<R> {
   const Visitor();
 
   R visitNode(Node node);
@@ skipping 649 matching lines @@
   final NodeList update;
 
   final Token forToken;
 
   For(this.initializer, this.conditionStatement, this.update, body,
       this.forToken) : super(body);
 
   For asFor() => this;
 
   Expression get condition {
-    if (conditionStatement is ExpressionStatement) {
-      return conditionStatement.asExpressionStatement().expression;
+    ExpressionStatement expressionStatement =
+        conditionStatement.asExpressionStatement();
+    if (expressionStatement != null) {
+      return expressionStatement.expression;
     } else {
       return null;
     }
   }
 
   accept(Visitor visitor) => visitor.visitFor(this);
 
   visitChildren(Visitor visitor) {
     if (initializer != null) initializer.accept(visitor);
     if (conditionStatement != null) conditionStatement.accept(visitor);
@@ skipping 1496 matching lines @@
 
   // VariableDefinitions.
   get metadata => null;
   get type => null;
 
   // Typedef.
   get typeParameters => null;
   get formals => null;
   get typedefKeyword => null;
 }