Add empty kernel-ssa test

tests/compiler/dart2js/kernel/helper.dart

+// Copyright (c) 2016, 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.
+
+import 'package:compiler/src/elements/elements.dart' show Element;
+import 'package:compiler/src/js_backend/backend.dart' show JavaScriptBackend;
+import 'package:compiler/src/commandline_options.dart' show Flags;
+import 'package:test/test.dart';
+
+import '../memory_compiler.dart';
+
+Future<String> compile(String code, {String entry: 'main',
+    bool useKernel: true}) async {
+  List<String> options = <String>[
+    Flags.disableTypeInference,
+    Flags.disableInlining,
+  ];
+  if (useKernel) options.add(Flags.useKernel);
+
+  if (entry != 'main' && !code.contains('main')) {
+    code = "$code\n\nmain() => $entry;";
+  }
+  CompilationResult result = await runCompiler(
+      memorySourceFiles: {'main.dart': code},
+      options: options);
+  expect(result.isSuccess, isTrue);
+  Compiler compiler = result.compiler;
+  Element element = compiler.mainApp.find(entry);
+  js.JavaScriptBackend backend = compiler.backend;
+  return backend.getGeneratedCode(element);
+}
+
+void check(String code, {String entry: 'main'}) async {
+  var original = await compile(code, entry: entry, useKernel: false);
+  var kernel = await compile(code, entry: entry, useKernel: true);
+  expect(original, kernel);
+}