Add support for Flutter SDK constraints.

lib/src/cached_package.dart

 // Copyright (c) 2014, 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:path/path.dart' as p;
 import 'package:pub_semver/pub_semver.dart';
 import 'package:yaml/yaml.dart';
 
 import 'barback/transformer_config.dart';
 import 'io.dart';
@@ skipping 53 matching lines @@
 /// A pubspec wrapper that reports no transformers.
 class _CachedPubspec implements Pubspec {
   final Pubspec _inner;
 
   YamlMap get fields => _inner.fields;
   String get name => _inner.name;
   Version get version => _inner.version;
   List<PackageDep> get dependencies => _inner.dependencies;
   List<PackageDep> get devDependencies => _inner.devDependencies;
   List<PackageDep> get dependencyOverrides => _inner.dependencyOverrides;
-  PubspecEnvironment get environment => _inner.environment;
+  VersionConstraint get dartSdkConstraint => _inner.dartSdkConstraint;
+  VersionConstraint get flutterSdkConstraint => _inner.flutterSdkConstraint;
   String get publishTo => _inner.publishTo;
   Map<String, String> get executables => _inner.executables;
   bool get isPrivate => _inner.isPrivate;
   bool get isEmpty => _inner.isEmpty;
   List<PubspecException> get allErrors => _inner.allErrors;
 
   List<Set<TransformerConfig>> get transformers => const [];
 
   _CachedPubspec(this._inner);
 }