Implement optimized mixin application in dart2js

tests/compiler/dart2js/kernel/compile_from_dill_test_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.
 
 // Helper to test compilation equivalence between source and .dill based
 // compilation.
 library dart2js.kernel.compile_from_dill_test_helper;
 
 import 'dart:async';
-import 'dart:io';
 
 import 'package:compiler/compiler_new.dart';
 import 'package:compiler/src/commandline_options.dart';
 import 'package:compiler/src/common.dart';
 import 'package:compiler/src/compiler.dart';
 import 'package:compiler/src/elements/elements.dart';
 import 'package:compiler/src/elements/types.dart';
 import 'package:compiler/src/kernel/element_map.dart';
 import 'package:compiler/src/kernel/kernel_backend_strategy.dart';
 import 'package:compiler/src/kernel/kernel_strategy.dart';
@@ skipping 200 matching lines @@
     {bool skipWarnings: false,
     bool skipErrors: false,
     bool verbose: false,
     List<String> options: const <String>[],
     bool expectAstEquivalence: false,
     bool expectIdenticalOutput: true}) async {
   enableDebugMode();
   EnumCreator.matchKernelRepresentationForTesting = true;
   Elements.usePatchedDart2jsSdkSorting = true;
 
-  Directory dir = await Directory.systemTemp.createTemp('dart2js-with-dill');
-  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);
+  entryPoint =
+      await createTemp(entryPoint, memorySourceFiles, printSteps: true);
 
   print('---- compile from ast ----------------------------------------------');
   DiagnosticCollector collector = new DiagnosticCollector();
   OutputCollector collector1 = new OutputCollector();
   Compiler compiler1 = compilerFor(
       entryPoint: entryPoint,
       diagnosticHandler: collector,
       outputProvider: collector1,
       options: <String>[]..addAll(commonOptions)..addAll(options));
   ElementResolutionWorldBuilder.useInstantiationMap = true;
@@ skipping 93 matching lines @@
       Map<String, BufferedOutputSink> map2 = collector2.outputMap[outputType];
       checkSets(map1.keys, map2.keys, 'output', equality);
       map1.forEach((String name, BufferedOutputSink output1) {
         BufferedOutputSink output2 = map2[name];
         Expect.stringEquals(output1.text, output2.text);
       });
     });
   }
   return ResultKind.success;
 }