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

tests/compiler/dart2js/deferred_not_in_main_test.dart

Index: tests/compiler/dart2js/deferred_not_in_main_test.dart
diff --git a/tests/compiler/dart2js/deferred_not_in_main_test.dart b/tests/compiler/dart2js/deferred_not_in_main_test.dart
index 1e5ed61c4617a0daffeee9de9978405c49f72554..80f2a230f9ddc31ea255754225a4404548aa80c7 100644
--- a/tests/compiler/dart2js/deferred_not_in_main_test.dart
+++ b/tests/compiler/dart2js/deferred_not_in_main_test.dart
@@ -21,7 +21,7 @@ void main() {
       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;