Refactor BackendHelpers to be reusageable with KernelWorldBuilder

tests/compiler/dart2js/map_tracer_test.dart

 // Copyright (c) 2014, 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.
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
 import 'package:compiler/src/types/types.dart' show MapTypeMask, TypeMask;
 
 import 'compiler_helper.dart';
 import 'type_mask_test_helper.dart';
@@ skipping 193 matching lines @@
   // Test preset map of <String,uint32>
   doTest('{presetKey : anInt}', "presetKey", "anInt");
   // Test preset map of <Double,uint32>
   doTest('{aDouble : anInt}', "aDouble", "anInt");
 }
 
 void doTest(String allocation, [String keyElement, String valueElement]) {
   Uri uri = new Uri(scheme: 'source');
   var compiler = compilerFor(generateTest(allocation), uri,
       expectedErrors: 0, expectedWarnings: 1);
-  compiler.closeResolution();
   asyncTest(() => compiler.run(uri).then((_) {
+        compiler.closeResolution();
         var keyType, valueType;
         var typesInferrer = compiler.globalInference.typesInferrerInternal;
         var closedWorld = typesInferrer.closedWorld;
         var commonMasks = closedWorld.commonMasks;
         var emptyType = new TypeMask.nonNullEmpty();
         var aKeyType =
             typesInferrer.getTypeOfElement(findElement(compiler, 'aKey'));
         if (keyElement != null) {
           keyType =
               typesInferrer.getTypeOfElement(findElement(compiler, keyElement));
@@ skipping 53 matching lines @@
         checkType('mapStoredInListButEscapes', K(commonMasks.dynamicType),
             V(commonMasks.dynamicType));
         checkType('mapStoredInMap', K(aKeyType), V(commonMasks.uint31Type));
         checkType('mapStoredInMapButEscapes', K(commonMasks.dynamicType),
             V(commonMasks.dynamicType));
 
         checkType('mapUnset', K(emptyType), V(emptyType));
         checkType('mapOnlySetWithConstraint', K(aKeyType), V(emptyType));
       }));
 }