Move common identifiers, names and selectors to a separate library.

tests/compiler/dart2js/deferred_not_in_main_test.dart

 // Copyright (c) 2014, 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 of the graph segmentation algorithm used by deferred loading
 // to determine which elements can be deferred and which libraries
 // much be included in the initial download (loaded eagerly).
 
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/compiler.dart';
 import 'package:expect/expect.dart';
 import 'memory_compiler.dart';
 
 void main() {
   asyncTest(() async {
     CompilationResult result =
         await runCompiler(memorySourceFiles: MEMORY_SOURCE_FILES);
     Compiler compiler = result.compiler;
 
     lookupLibrary(name) {
       return compiler.libraryLoader.lookupLibrary(Uri.parse(name));
     }
 
-    var main = compiler.mainApp.find(Compiler.MAIN);
+    var main = compiler.mainFunction;
     var outputUnitForElement = compiler.deferredLoadTask.outputUnitForElement;
 
     var mainOutputUnit = compiler.deferredLoadTask.mainOutputUnit;
     var backend = compiler.backend;
     var classes = backend.emitter.neededClasses;
     var lib1 = lookupLibrary("memory:lib1.dart");
     var lib2 = lookupLibrary("memory:lib2.dart");
     var foo1 = lib1.find("foo1");
     var foo2 = lib2.find("foo2");
 
@@ skipping 23 matching lines @@
 void foo1() {
   lib1.loadLibrary().then((_) => lib2.foo2());
 }
 """,
   "lib2.dart":"""
 library lib2;
 
 void foo2() {}
 """,
 };