add a profile() method (#11443)

* add a profile() method

* add todos

packages/flutter/lib/foundation.dart

@@ -42,6 +42,7 @@ export 'src/foundation/licenses.dart';
 export 'src/foundation/observer_list.dart';
 export 'src/foundation/platform.dart';
 export 'src/foundation/print.dart';
+export 'src/foundation/profile.dart';
 export 'src/foundation/serialization.dart';
 export 'src/foundation/synchronous_future.dart';
 export 'src/foundation/tree_diagnostics_mixin.dart';

packages/flutter/lib/src/foundation/profile.dart

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:ui';
+
+/// Whether we've been built in release mode.
+const bool _kReleaseMode = const bool.fromEnvironment("dart.vm.product");
+
+/// When running in profile mode (or debug mode), invoke the given function.
+///
+/// In release mode, the function is not invoked.
+// TODO(devoncarew): Going forward, we'll want the call to profile() to be tree-shaken out.
+void profile(VoidCallback function) {
+  if (_kReleaseMode)
+    return;
+  function();
+}

packages/flutter/test/foundation/profile_test.dart

+// Copyright 2017 The Chromium Authors. 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:flutter/foundation.dart';
+import 'package:test/test.dart';
+
+// We run our tests in debug mode, to this will always evaluate to false...
+const bool isReleaseMode = const bool.fromEnvironment("dart.vm.product");
+
+void main() {
+  // TODO(devoncarew): This test - while very nice - isn't testing what we really want to know:
+  // that the code in the `profile` closure is omitted in release mode.
+  test("profile invokes its closure in debug or profile mode", () {
+    int count = 0;
+    profile(() {
+      count++;
+    });
+    expect(count, isReleaseMode ? 0 : 1);
+  });
+}