Add FunctionEntity.asyncMarker and refactor SourceInformationStrategy to avoid ResolvedAst

pkg/compiler/lib/src/js_backend/impact_transformer.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 js_backend.backend.impact_transformer;
 
 import '../common.dart';
 import '../common_elements.dart';
 import '../common/backend_api.dart' show ImpactTransformer;
 import '../common/codegen.dart' show CodegenImpact;
 import '../common/resolution.dart' show ResolutionImpact;
 import '../constants/expressions.dart';
 import '../common_elements.dart' show ElementEnvironment;
-import '../elements/elements.dart' show AsyncMarker;
 import '../elements/entities.dart';
 import '../elements/resolution_types.dart' show Types;
 import '../elements/types.dart';
 import '../native/enqueue.dart';
 import '../native/native.dart' as native;
 import '../options.dart';
 import '../universe/feature.dart';
 import '../universe/use.dart'
     show StaticUse, StaticUseKind, TypeUse, TypeUseKind;
 import '../universe/world_impact.dart' show TransformedWorldImpact, WorldImpact;
@@ skipping 442 matching lines @@
           _impacts.asyncStarBody
               .registerImpact(transformed, _elementEnvironment);
           break;
       }
     }
 
     // TODO(johnniwinther): Remove eager registration.
     return transformed;
   }
 }