Remove Compiler.mainApp and Compiler.mainFunction

tests/compiler/dart2js/kernel/helper.dart

 // Copyright (c) 2016, 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 'dart:async';
 
 import 'package:compiler/src/compiler.dart' show Compiler;
 import 'package:compiler/src/elements/elements.dart'
     show Element, LibraryElement;
 import 'package:compiler/src/js_backend/backend.dart' as js
@@ skipping 15 matching lines @@
   if (useKernel) options.add(Flags.useKernel);
   options.addAll(extraOptions);
 
   if (lookup is String && lookup != 'main' && !code.contains('main')) {
     code = "$code\n\nmain() => $lookup;";
   }
   CompilationResult result = await runCompiler(
       memorySourceFiles: {'main.dart': code}, options: options);
   expect(result.isSuccess, isTrue);
   Compiler compiler = result.compiler;
-  LibraryElement mainApp = compiler.mainApp;
+  LibraryElement mainApp =
+      compiler.frontendStrategy.elementEnvironment.mainLibrary;
   Element element;
   if (lookup is String) {
     element = mainApp.find(lookup);
   } else {
     element = lookup(compiler);
   }
   js.JavaScriptBackend backend = compiler.backend;
   return backend.getGeneratedCode(element);
 }
 
@@ skipping 13 matching lines @@
       useKernel: false,
       disableTypeInference: disableTypeInference,
       extraOptions: extraOptions);
   var kernel = await compile(code,
       lookup: lookup,
       useKernel: true,
       disableTypeInference: disableTypeInference,
       extraOptions: extraOptions);
   expect(kernel, original);
 }