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

tests/compiler/dart2js/use_checks_test.dart

Index: tests/compiler/dart2js/use_checks_test.dart
diff --git a/tests/compiler/dart2js/use_checks_test.dart b/tests/compiler/dart2js/use_checks_test.dart
index 24013ad49347987b92a91d65ae63b837ca8e6094..26d81c659fb7e017784a870b8134a3be92285719 100644
--- a/tests/compiler/dart2js/use_checks_test.dart
+++ b/tests/compiler/dart2js/use_checks_test.dart
@@ -28,7 +28,7 @@ main() {
         memorySourceFiles: MEMORY_SOURCE_FILES,
         options: ['--enable-checked-mode']);
     var compiler = result.compiler;
-    var element = compiler.mainApp.findExported('main');
+    var element = compiler.mainFunction;
     var code = compiler.backend.assembleCode(element);
     Expect.isTrue(code.contains('+'), code);
   });