Add BackendStrategy

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 'compiler.dart';
+import 'js_emitter/sorter.dart';
+import 'world.dart';
+
+/// Strategy pattern that defines the element model used in type inference
+/// and code generation.
+abstract class BackendStrategy {
+  /// Create the [ClosedWorldRefiner] for [closedWorld].
+  ClosedWorldRefiner createClosedWorldRefiner(ClosedWorld closedWorld);
+
+  /// Create closure classes for local functions.
+  void convertClosures(ClosedWorldRefiner closedWorldRefiner);
+
+  /// The [Sorter] used for sorting elements in the generated code.
+  Sorter get sorter;
+}
+
+/// Strategy for using the [Element] model from the resolver as the backend
+/// model.
+class ElementBackendStrategy implements BackendStrategy {
+  final Compiler _compiler;
+
+  ElementBackendStrategy(this._compiler);
+
+  ClosedWorldRefiner createClosedWorldRefiner(ClosedWorldImpl closedWorld) =>
+      closedWorld;
+
+  Sorter get sorter => const ElementSorter();
+
+  void convertClosures(ClosedWorldRefiner closedWorldRefiner) {
+    _compiler.closureToClassMapper.createClosureClasses(closedWorldRefiner);
+  }
+}