Remove Compiler.mainApp and Compiler.mainFunction

tests/compiler/dart2js/assert_message_throw_test.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 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/commandline_options.dart';
 import 'package:compiler/src/compiler.dart';
 import 'package:compiler/src/elements/elements.dart';
 import 'package:compiler/src/types/masks.dart';
 import 'package:compiler/src/world.dart' show ClosedWorld;
@@ skipping 45 matching lines @@
   asyncTest(() async {
     CompilationResult result = await runCompiler(
         entryPoint: Uri.parse('memory:main.dart'),
         memorySourceFiles: {'main.dart': SOURCE},
         options: [Flags.enableCheckedMode, Flags.enableAssertMessage]);
     Compiler compiler = result.compiler;
     ClosedWorld closedWorld =
         compiler.resolutionWorldBuilder.closedWorldForTesting;
 
     void check(String methodName, TypeMask expectedReturnType) {
-      LibraryElement mainApp = compiler.mainApp;
+      LibraryElement mainApp = closedWorld.elementEnvironment.mainLibrary;
       MemberElement element = mainApp.find(methodName);
       TypeMask typeMask = simplify(
           compiler.globalInference.results.resultOfMember(element).returnType,
           closedWorld);
       Expect.equals(expectedReturnType, typeMask,
           "Unexpected return type on method '$methodName'.");
     }
 
     check('test0', closedWorld.commonMasks.growableListType);
     check('test1', closedWorld.commonMasks.nullType);
     check('test2', closedWorld.commonMasks.uint31Type.nullable());
     check('test3', closedWorld.commonMasks.intType);
   });
 }