Remove Compiler.mainApp and Compiler.mainFunction

tests/compiler/dart2js/deferred_follow_implicit_super_regression_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';
 
 import 'package:compiler/src/compiler.dart' as dart2js;
 
 void main() {
   asyncTest(() async {
     CompilationResult result =
         await runCompiler(memorySourceFiles: MEMORY_SOURCE_FILES);
     dart2js.Compiler compiler = result.compiler;
 
     lookupLibrary(name) {
       return compiler.libraryLoader.lookupLibrary(Uri.parse(name));
     }
 
-    var main = compiler.mainFunction;
+    var main = compiler.frontendStrategy.elementEnvironment.mainFunction;
     Expect.isNotNull(main, "Could not find 'main'");
     compiler.deferredLoadTask.onResolutionComplete(
         main, compiler.resolutionWorldBuilder.closedWorldForTesting);
 
     var outputUnitForElement = compiler.deferredLoadTask.outputUnitForElement;
 
     var lib = lookupLibrary("memory:lib.dart");
     var a = lib.find("a");
     var b = lib.find("b");
     var c = lib.find("c");
@@ skipping 74 matching lines @@
 class D2 {
   D2(x) {
     d();
   }
 }
 
 // Implicit redirecting "super" call with a parameter via mixin.
 class D3 = D2 with D1;
 """,
 };