Add support for Flutter SDK constraints.

lib/src/validator/sdk_constraint.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 'dart:async';
 
 import '../entrypoint.dart';
 import '../validator.dart';
 
 /// A validator that validates that a package's SDK constraint doesn't use the
 /// "^" syntax.
 class SdkConstraintValidator extends Validator {
   SdkConstraintValidator(Entrypoint entrypoint)
     : super(entrypoint);
 
   Future validate() async {
-    var constraint = entrypoint.root.pubspec.environment.sdkVersion;
+    var constraint = entrypoint.root.pubspec.dartSdkConstraint;
     if (!constraint.toString().startsWith("^")) return;
 
     errors.add(
         "^ version constraints aren't allowed for SDK constraints since "
           "older versions of pub don't support them.\n"
         "Expand it manually instead:\n"
         "\n"
         "environment:\n"
         "  sdk: \">=${constraint.min} <${constraint.max}\"");
   }
 }