Remove Compiler.mainApp and Compiler.mainFunction

tests/compiler/dart2js/constant_expression_evaluate_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 dart2js.constants.expressions.evaluate_test;
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 import 'package:compiler/src/common/backend_api.dart';
@@ skipping 258 matching lines @@
   data.constants.forEach((ConstantData constantData) {
     String name = 'c${constants.length}';
     sb.write('const $name = ${constantData.code};\n');
     constants[name] = constantData;
   });
   sb.write('main() {}\n');
   String source = sb.toString();
   CompilationResult result = await runCompiler(
       memorySourceFiles: {'main.dart': source}, options: ['--analyze-all']);
   Compiler compiler = result.compiler;
-  var library = compiler.mainApp;
+  var library = compiler.frontendStrategy.elementEnvironment.mainLibrary;
   constants.forEach((String name, ConstantData data) {
     FieldElement field = library.localLookup(name);
     ConstantExpression constant = field.constant;
     data.expectedValues.forEach((Map<String, String> env, String expectedText) {
       EvaluationEnvironment environment = new MemoryEnvironment(compiler, env);
       ConstantValue value =
           constant.evaluate(environment, DART_CONSTANT_SYSTEM);
       String valueText = value.toStructuredText();
       Expect.equals(
           expectedText,
           valueText,
           "Unexpected value '${valueText}' for constant "
           "`${constant.toDartText()}`, expected '${expectedText}'.");
     });
   });
 }