Remove Compiler.mainApp and Compiler.mainFunction

tests/compiler/dart2js/use_checks_test.dart

 // Copyright (c) 2014, 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:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
 import 'memory_compiler.dart';
 
 const MEMORY_SOURCE_FILES = const {
   'main.dart': '''
@@ skipping 10 matching lines @@
 }
 ''',
 };
 
 main() {
   asyncTest(() async {
     var result = await runCompiler(
         memorySourceFiles: MEMORY_SOURCE_FILES,
         options: ['--enable-checked-mode']);
     var compiler = result.compiler;
-    var element = compiler.mainFunction;
+    var element = compiler.frontendStrategy.elementEnvironment.mainFunction;
     var code = compiler.backend.getGeneratedCode(element);
     Expect.isTrue(code.contains('+'), code);
   });
 }