Remove Compiler.mainApp and Compiler.mainFunction

tests/compiler/dart2js/constant_expression_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.
 
 library constant_expression_test;
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 import 'package:compiler/src/constants/expressions.dart';
@@ skipping 192 matching lines @@
     String name = 'c${constants.length}';
     sb.write('const $name = ${constantData.code};\n');
     constants[name] = constantData;
   });
   sb.write('main() {}\n');
   String source = sb.toString();
   CompilationResult result = await runCompiler(
       memorySourceFiles: {'main.dart': source}, options: ['--analyze-all']);
   Compiler compiler = result.compiler;
   MemoryEnvironment environment = new MemoryEnvironment(compiler);
-  var library = compiler.mainApp;
+  var library = compiler.frontendStrategy.elementEnvironment.mainLibrary;
   constants.forEach((String name, ConstantData data) {
     FieldElement field = library.localLookup(name);
     var constant = field.constant;
     Expect.equals(
         data.kind,
         constant.kind,
         "Unexpected kind '${constant.kind}' for constant "
         "`${constant.toDartText()}`, expected '${data.kind}'.");
     Expect.equals(
         data.text,
@@ skipping 22 matching lines @@
         String expected = data.fields[name];
         Expect.equals(
             expected,
             expression,
             "Unexpected field expression ${expression} for field '$name' in "
             "constant `${constant.toDartText()}`, expected '${expected}'.");
       });
     }
   });
 }