Remove Compiler.mainApp and Compiler.mainFunction

tests/compiler/dart2js/deferred_dont_inline_deferred_globals_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.
 
 // Test that the additional runtime type support is output to the right
 // Files when using deferred loading.
 
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/compiler_new.dart';
 import 'package:compiler/src/compiler.dart';
 import 'package:expect/expect.dart';
 import 'memory_compiler.dart';
 import 'output_collector.dart';
 
 void main() {
   OutputCollector collector = new OutputCollector();
   asyncTest(() async {
     CompilationResult result = await runCompiler(
         memorySourceFiles: MEMORY_SOURCE_FILES, outputProvider: collector);
     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 lib1 = lookupLibrary("memory:lib1.dart");
     var foo1 = lib1.find("finalVar");
     var ou_lib1 = outputUnitForElement(foo1);
 
@@ skipping 25 matching lines @@
     print(lib1.globalVar);
   });
 }
 """,
   "lib1.dart": """
 import "main.dart" as main;
 final finalVar = "string1";
 var globalVar = "string2";
 """
 };