Remove Compiler.mainApp and Compiler.mainFunction

tests/compiler/dart2js/container_mask_equal_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.
 
 // Regression test for dart2js that used to have a bogus
 // implementation of var.== and
 // var.hashCode.
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
@@ skipping 15 matching lines @@
 ''',
 };
 
 main() {
   asyncTest(() async {
     var result = await runCompiler(memorySourceFiles: MEMORY_SOURCE_FILES);
     var compiler = result.compiler;
     var typesInferrer = compiler.globalInference.typesInferrerInternal;
     var closedWorld = typesInferrer.closedWorld;
 
-    var element = compiler.mainApp.find('a');
+    var element =
+        compiler.frontendStrategy.elementEnvironment.mainLibrary.find('a');
     var mask1 = typesInferrer.getReturnTypeOfElement(element);
 
-    element = compiler.mainApp.find('b');
+    element =
+        compiler.frontendStrategy.elementEnvironment.mainLibrary.find('b');
     var mask2 = typesInferrer.getReturnTypeOfElement(element);
 
-    element = compiler.mainApp.find('c');
+    element =
+        compiler.frontendStrategy.elementEnvironment.mainLibrary.find('c');
     var mask3 = typesInferrer.getReturnTypeOfElement(element);
 
-    element = compiler.mainApp.find('d');
+    element =
+        compiler.frontendStrategy.elementEnvironment.mainLibrary.find('d');
     var mask4 = typesInferrer.getReturnTypeOfElement(element);
 
     Expect.notEquals(
         mask1.union(mask2, closedWorld), mask3.union(mask4, closedWorld));
   });
 }