Add FunctionEntity.asyncMarker and refactor SourceInformationStrategy to avoid ResolvedAst

pkg/compiler/lib/src/backend_strategy.dart

 // Copyright (c) 2017, 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 dart2js.backend_strategy;
 
 import 'enqueue.dart';
 import 'io/source_information.dart';
 import 'js_backend/js_backend.dart';
 import 'js_backend/native_data.dart';
@@ skipping 19 matching lines @@
       NativeBasicData nativeBasicData,
       ClosedWorld closedWorld,
       SelectorConstraintsStrategy selectorConstraintsStrategy);
 
   /// Creates the [WorkItemBuilder] used by the codegen enqueuer.
   WorkItemBuilder createCodegenWorkItemBuilder(ClosedWorld closedWorld);
 
   /// Creates the [SsaBuilderTask] used for the element model.
   SsaBuilderTask createSsaBuilderTask(JavaScriptBackend backend,
       SourceInformationStrategy sourceInformationStrategy);
+
+  /// Returns the [SourceInformationStrategy] use for the element model.
+  SourceInformationStrategy get sourceInformationStrategy;
 }