Remove Compiler.mainApp and Compiler.mainFunction

tests/compiler/dart2js/inference/inference_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.
 
 import 'package:compiler/src/common.dart';
 import 'package:compiler/src/common_elements.dart';
 import 'package:compiler/src/compiler.dart';
 import 'package:compiler/src/elements/elements.dart';
 import 'package:compiler/src/elements/entities.dart';
 import 'package:compiler/src/resolution/tree_elements.dart';
@@ skipping 13 matching lines @@
 /// annotation. Any [Id] left in the map will be reported as missing.
 checkCode(String annotatedCode, CheckMemberFunction checkMember,
     {List<String> options: const <String>[]}) async {
   AnnotatedCode code = new AnnotatedCode.fromText(annotatedCode, '/*', '*/');
   Map<Id, String> expectedMap = computeExpectedMap(code);
   Compiler compiler = compilerFor(
       memorySourceFiles: {'main.dart': code.sourceCode}, options: options);
   compiler.stopAfterTypeInference = true;
   Uri mainUri = Uri.parse('memory:main.dart');
   await compiler.run(mainUri);
-  LibraryElement mainApp = compiler.mainApp;
+  LibraryElement mainApp =
+      compiler.frontendStrategy.elementEnvironment.mainLibrary;
   mainApp.forEachLocalMember((member) {
     if (member.isClass) {
       member.forEachLocalMember((member) {
         checkMember(compiler, expectedMap, member);
       });
     } else if (member.isTypedef) {
       // Skip.
     } else {
       checkMember(compiler, expectedMap, member);
     }
@@ skipping 128 matching lines @@
   visitSend(Send node) {
     checkSend(node);
     visitNode(node);
   }
 
   visitSendSet(SendSet node) {
     checkSend(node);
     visitNode(node);
   }
 }