Add FunctionEntity.asyncMarker and refactor SourceInformationStrategy to avoid ResolvedAst

pkg/compiler/lib/src/ssa/rasta_ssa_builder_task.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.
 import '../elements/elements.dart';
 import '../io/source_information.dart';
 import '../js_backend/backend.dart' show JavaScriptBackend;
 import '../js_backend/element_strategy.dart' show ElementCodegenWorkItem;
 import '../kernel/kernel.dart';
 import '../world.dart';
 import 'builder.dart';
@@ skipping 25 matching lines @@
           new KernelAstTypeInferenceMap(astAdapter);
       KernelSsaBuilder builder = new KernelSsaBuilder(
           element,
           astAdapter.getInitialKernelNode(element),
           backend.compiler,
           astAdapter,
           typeInferenceMap,
           closedWorld,
           work.registry,
           backend.compiler.closureToClassMapper,
-          sourceInformationFactory.createBuilderForContext(resolvedAst),
+          sourceInformationFactory.createBuilderForContext(work.element),
           resolvedAst.kind == ResolvedAstKind.PARSED ? resolvedAst.node : null,
           targetIsConstructorBody: element is ConstructorBodyElement);
       HGraph graph = builder.build();
 
       if (backend.tracer.isEnabled) {
         String name;
         if (element.isClassMember) {
           String className = element.enclosingClass.name;
           String memberName = element.name;
           name = "$className.$memberName";
           if (element.isGenerativeConstructorBody) {
             name = "$name (body)";
           }
         } else {
           name = "${element.name}";
         }
         backend.tracer.traceCompilation(name);
         backend.tracer.traceGraph('builder', graph);
       }
 
       return graph;
     });
   }
 }