Typecheck return with respect to async.

pkg/compiler/lib/src/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 1048 matching lines @@
   final Node expression;
   final Token beginToken;
   final Token endToken;
 
   Return(this.beginToken, this.endToken, this.expression);
 
   Return asReturn() => this;
 
   bool get hasExpression => expression != null;
 
+  /// `true` if this return is of the form `=> e;`.
+  bool get isArrowBody => beginToken.info == FUNCTION_INFO;
+
   accept(Visitor visitor) => visitor.visitReturn(this);
 
   visitChildren(Visitor visitor) {
     if (expression != null) expression.accept(visitor);
   }
 
   Token getBeginToken() => beginToken;
 
   Token getEndToken() {
     if (endToken == null) return expression.getEndToken();
@@ skipping 1215 matching lines @@
 
   // VariableDefinitions.
   get metadata => null;
   get type => null;
 
   // Typedef.
   get typeParameters => null;
   get formals => null;
   get typedefKeyword => null;
 }