Add support for Flutter SDK constraints.

lib/src/cached_package.dart

Index: lib/src/cached_package.dart
diff --git a/lib/src/cached_package.dart b/lib/src/cached_package.dart
index c67a3c1f9ccf2e06cc575b6411aa100c9c170940..bd6fec0e28fb242ddca82a1dc98b146e7d2f2aa4 100644
--- a/lib/src/cached_package.dart
+++ b/lib/src/cached_package.dart
@@ -71,7 +71,8 @@ class _CachedPubspec implements Pubspec {
   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;