Refactor BackendHelpers to be reusageable with KernelWorldBuilder

pkg/compiler/lib/src/inferrer/builder.dart

 // Copyright (c) 2013, 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 simple_types_inferrer;
 
 import '../closure.dart' show ClosureClassMap;
 import '../common.dart';
 import '../common/names.dart' show Identifiers, Selectors;
 import '../compiler.dart' show Compiler;
@@ skipping 2894 matching lines @@
 
   TypeInformation visitAsyncForIn(ast.AsyncForIn node) {
     TypeInformation expressionType = visit(node.expression);
 
     Selector currentSelector = Selectors.current;
     TypeMask currentMask = inTreeData.typeOfIteratorCurrent(node);
     Selector moveNextSelector = Selectors.moveNext;
     TypeMask moveNextMask = inTreeData.typeOfIteratorMoveNext(node);
 
     js.JavaScriptBackend backend = compiler.backend;
-    Element ctor = backend.helpers.streamIteratorConstructor;
+    ConstructorElement ctor = backend.helpers.streamIteratorConstructor;
 
     /// Synthesize a call to the [StreamIterator] constructor.
     TypeInformation iteratorType = handleStaticSend(
         node, null, null, ctor, new ArgumentsTypes([expressionType], null));
 
     return handleForInLoop(node, iteratorType, currentSelector, currentMask,
         moveNextSelector, moveNextMask);
   }
 
   TypeInformation visitSyncForIn(ast.SyncForIn node) {
     TypeInformation expressionType = visit(node.expression);
     Selector iteratorSelector = Selectors.iterator;
     TypeMask iteratorMask = inTreeData.typeOfIterator(node);
     Selector currentSelector = Selectors.current;
     TypeMask currentMask = inTreeData.typeOfIteratorCurrent(node);
     Selector moveNextSelector = Selectors.moveNext;
     TypeMask moveNextMask = inTreeData.typeOfIteratorMoveNext(node);
 
     TypeInformation iteratorType = handleDynamicSend(node, iteratorSelector,
         iteratorMask, expressionType, new ArgumentsTypes.empty());
 
     return handleForInLoop(node, iteratorType, currentSelector, currentMask,
         moveNextSelector, moveNextMask);
   }
 }