Remove Compiler.mainApp and Compiler.mainFunction

tests/compiler/dart2js/array_tracing_mirror_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.
 
 // Make sure that limiting mirrors through @MirrorsUsed does not
 // affect optimizations done on arrays.
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
 import 'memory_compiler.dart';
@@ skipping 16 matching lines @@
   field.invoke(#clear, []);
   return array.length;
 }
 ''',
 };
 
 main() {
   asyncTest(() async {
     var result = await runCompiler(memorySourceFiles: MEMORY_SOURCE_FILES);
     var compiler = result.compiler;
-    var element = compiler.mainFunction;
+    var element = compiler.frontendStrategy.elementEnvironment.mainFunction;
     var code = compiler.backend.getGeneratedCode(element);
     Expect.isTrue(code.contains('return 2'), "Unexpected code:\n$code");
   });
 }