Add support for Flutter SDK constraints.

lib/src/flutter.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 'dart:io';
+
+import 'package:path/path.dart' as p;
+import 'package:pub_semver/pub_semver.dart';
+
+import 'io.dart';
+
+/// Whether the Flutter SDK is available.
+final bool isAvailable = Platform.environment.containsKey("FLUTTER_ROOT");
+
+/// The path to the root directory of the Flutter SDK.
+final String rootDirectory = Platform.environment["FLUTTER_ROOT"];
+
+/// The Flutter SDK's version number, or `null` if the Flutter SDK is
+/// unavailable.
+final version = () {
+  if (!isAvailable) return null;
+
+  return new Version.parse(
+      readTextFile(p.join(rootDirectory, "version")).trim());
+}();