Remove Compiler.mainApp and Compiler.mainFunction

tests/compiler/dart2js/benign_error_test.dart

 // Copyright (c) 2015, 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 benign error do not prevent compilation.
 
 import 'memory_compiler.dart';
 
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/compiler.dart';
@@ skipping 16 matching lines @@
       example = {'main.dart': example};
     }
     DiagnosticCollector collector = new DiagnosticCollector();
     CompilationResult result = await runCompiler(
         memorySourceFiles: example, diagnosticHandler: collector);
     Expect.isTrue(result.isSuccess);
     Expect
         .isTrue(collector.errors.any((message) => message.messageKind == kind));
     Compiler compiler = result.compiler;
     JavaScriptBackend backend = compiler.backend;
-    Expect.isNotNull(backend.generatedCode[compiler.mainFunction]);
+    Expect.isNotNull(backend.generatedCode[
+        compiler.frontendStrategy.elementEnvironment.mainFunction]);
   }
 }