Introduce JsStrategy skeleton

tests/compiler/dart2js/kernel/compiler_helper.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.
 
 // Partial test that the closed world computed from [WorldImpact]s derived from
 // kernel is equivalent to the original computed from resolution.
 library dart2js.kernel.compiler_helper;
 
 import 'dart:async';
 import 'dart:io';
@@ skipping 123 matching lines @@
       : super(elementMap, null, null, reporter, measurer);
 
   Future<LoadedLibraries> loadLibrary(Uri resolvedUri,
       {bool skipFileWithPartOfTag: false}) async {
     return createLoadedLibraries(program);
   }
 }
 
 Future<Compiler> compileWithDill(
     Uri entryPoint, Map<String, String> memorySourceFiles, List<String> options,
-    {bool printSteps: false, CompilerOutput compilerOutput}) async {
+    {bool printSteps: false,
+    CompilerOutput compilerOutput,
+    void beforeRun(Compiler compiler)}) async {
   if (memorySourceFiles.isNotEmpty) {
     Directory dir = await Directory.systemTemp.createTemp('dart2js-with-dill');
     if (printSteps) {
       print('--- create temp directory $dir -------------------------------');
     }
     memorySourceFiles.forEach((String name, String source) {
       new File.fromUri(dir.uri.resolve(name)).writeAsStringSync(source);
     });
     entryPoint = dir.uri.resolve(entryPoint.path);
   }
@@ skipping 12 matching lines @@
 
   if (printSteps) {
     print('---- compile from dill $dillFile ---------------------------------');
   }
   Compiler compiler = compilerFor(
       entryPoint: dillFile,
       options: [Flags.loadFromDill]..addAll(options),
       outputProvider: compilerOutput);
   ElementResolutionWorldBuilder.useInstantiationMap = true;
   compiler.resolution.retainCachesForTesting = true;
+  if (beforeRun != null) {
+    beforeRun(compiler);
+  }
   await compiler.run(dillFile);
   return compiler;
 }