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 createClosureClasses(ClosedWorldRefiner closedWorldRefiner);

[Emily Fortuna, 2017/05/02 18:06:39]

this name is still in line with the "old way" of thinking about creating
closures in terms of fake elements. Perhaps we should just call this function
"rewriteClosures" or "convertClosures" or something instead?

[Siggi Cherem (dart-lang), 2017/05/02 20:13:17]

+1 :)

[Johnni Winther, 2017/05/03 08:25:44]

[convertClosures] it is.

+
+  /// 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 createClosureClasses(ClosedWorldRefiner closedWorldRefiner) {
+    _compiler.closureToClassMapper.createClosureClasses(closedWorldRefiner);
+  }
+}