Add FunctionEntity.asyncMarker and refactor SourceInformationStrategy to avoid ResolvedAst

pkg/compiler/lib/src/backend_strategy.dart

Index: pkg/compiler/lib/src/backend_strategy.dart
diff --git a/pkg/compiler/lib/src/backend_strategy.dart b/pkg/compiler/lib/src/backend_strategy.dart
index 8de92a4ca10a3f5ba8e5b0a6b4ef6fe8cc828dd1..fe1ebedd7517b5ab46c1ea7e12d914800dc14ade 100644
--- a/pkg/compiler/lib/src/backend_strategy.dart
+++ b/pkg/compiler/lib/src/backend_strategy.dart
@@ -37,4 +37,7 @@ abstract class BackendStrategy {
   /// 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;
 }