Fix several bugs in closure conversion.

pkg/kernel/lib/transformations/closure/invalidate_closures.dart

 // Copyright (c) 2016, 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.
 
 library kernel.transformations.closure.invalidate;
 
 import '../../ast.dart';
 
 class InvalidateClosures extends Transformer {
   FunctionDeclaration visitFunctionDeclaration(FunctionDeclaration node) {
     invalidate(node.function);
     return node;
   }
 
   FunctionExpression visitFunctionExpression(FunctionExpression node) {
     invalidate(node.function);
     return node;
   }
 
   void invalidate(FunctionNode function) {
+    if (function.asyncMarker != AsyncMarker.Sync) return;
     var position = function.location;
     function.body = new ExpressionStatement(new Throw(
         new StringLiteral("Calling unconverted closure at $position")))
       ..parent = function;
   }
 }