Implement optimized mixin application in dart2js

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 134 matching lines @@
   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);
   }
+  return entryPoint;
+}
+
+Future generateDill(Uri entryPoint, Map<String, String> memorySourceFiles,
+    {bool printSteps: false}) async {
+  entryPoint =
+      await createTemp(entryPoint, memorySourceFiles, printSteps: printSteps);
   if (printSteps) {
     print('---- generate dill -----------------------------------------------');
   }
 
   Uri dillFile = Uri.parse('$entryPoint.dill');
   String buildDir = Platform.isMacOS ? 'xcodebuild' : 'out';
   String configuration =
       Platform.environment['DART_CONFIGURATION'] ?? 'ReleaseX64';
   await generate.main([
     '--platform=$buildDir/$configuration/patched_dart2js_sdk/platform.dill',
     '--out=${uriPathToNative(dillFile.path)}',
     '${entryPoint.path}',
   ]);
   return dillFile;
 }
 
 Future<Compiler> compileWithDill(
     Uri entryPoint, Map<String, String> memorySourceFiles, List<String> options,
     {bool printSteps: false,
     CompilerOutput compilerOutput,
     void beforeRun(Compiler compiler)}) async {
   Uri dillFile =
-      await createTemp(entryPoint, memorySourceFiles, printSteps: printSteps);
+      await generateDill(entryPoint, memorySourceFiles, printSteps: printSteps);
 
   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;
 }