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

tests/compiler/dart2js/dictionary_types_test.dart

Index: tests/compiler/dart2js/dictionary_types_test.dart
diff --git a/tests/compiler/dart2js/dictionary_types_test.dart b/tests/compiler/dart2js/dictionary_types_test.dart
index af56b543e204fd45153113ef6b55a719e0a426e4..11734e1247b87b52423f8fafd74dd6630bf281b5 100644
--- a/tests/compiler/dart2js/dictionary_types_test.dart
+++ b/tests/compiler/dart2js/dictionary_types_test.dart
@@ -146,7 +146,7 @@ compileAndTest(source, checker, {createCode: false}) async {
   if (!createCode) {
     checker(typesTask, getType, compiler);
   } else {
-    var element = compiler.mainApp.findExported('main');
+    var element = compiler.mainFunction;
     var code = compiler.backend.assembleCode(element);
     checker(code);
   }