Remove Compiler.mainApp and Compiler.mainFunction

tests/compiler/dart2js/deferred_load_graph_segmentation_test.dart

 // Copyright (c) 2013, 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.
 
 // Test of the graph segmentation algorithm used by deferred loading
 // to determine which elements can be deferred and which libraries
 // much be included in the initial download (loaded eagerly).
 
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/compiler.dart';
 import 'package:compiler/src/deferred_load.dart';
 import 'package:expect/expect.dart';
 import 'memory_compiler.dart';
 
 void main() {
   asyncTest(() async {
     CompilationResult result =
         await runCompiler(memorySourceFiles: MEMORY_SOURCE_FILES);
     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'");
 
     var outputUnitForElement = compiler.deferredLoadTask.outputUnitForElement;
 
     var mainOutputUnit = compiler.deferredLoadTask.mainOutputUnit;
     var backend = compiler.backend;
     var classes = backend.emitter.neededClasses;
     var inputElement = classes.where((e) => e.name == 'InputElement').single;
     var lib1 = lookupLibrary("memory:lib1.dart");
     var foo1 = lib1.find("foo1");
@@ skipping 105 matching lines @@
 
 bar1() {
   return "hello";
 }
 
 bar2() {
   return 2;
 }
 """,
 };