Add support for Flutter SDK constraints.

lib/src/validator/dependency.dart

 // Copyright (c) 2012, 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 'package:pub_semver/pub_semver.dart';
 
 import '../entrypoint.dart';
 import '../log.dart' as log;
@@ skipping 11 matching lines @@
 /// The range of all pub versions that do support `^` version constraints.
 ///
 /// This is intersected with the user's SDK constraint to provide a suggested
 /// constraint.
 final _postCaretPubVersions = new VersionConstraint.parse("^1.8.0");
 
 /// A validator that validates a package's dependencies.
 class DependencyValidator extends Validator {
   /// Whether the SDK constraint guarantees that `^` version constraints are
   /// safe.
-  bool get _caretAllowed => entrypoint.root.pubspec.environment.sdkVersion
+  bool get _caretAllowed => entrypoint.root.pubspec.dartSdkConstraint
       .intersect(_preCaretPubVersions).isEmpty;
 
   DependencyValidator(Entrypoint entrypoint)
     : super(entrypoint);
 
   Future validate() async {
     var caretDeps = [];
 
     for (var dependency in entrypoint.root.pubspec.dependencies) {
       if (dependency.source is! HostedSource) {
@@ skipping 124 matching lines @@
         'dependencies:\n'
         '  ${dep.name}: $constraint\n'
         '\n'
         'Without an upper bound, you\'re promising to support '
             '${log.bold("all")} future versions of ${dep.name}.');
   }
 
   /// Emits an error for any version constraints that use `^` without an
   /// appropriate SDK constraint.
   void _errorAboutCaretConstraints(List<PackageDep> caretDeps) {
-    var newSdkConstraint = entrypoint.root.pubspec.environment.sdkVersion
+    var newSdkConstraint = entrypoint.root.pubspec.dartSdkConstraint
         .intersect(_postCaretPubVersions);
 
     if (newSdkConstraint.isEmpty) newSdkConstraint = _postCaretPubVersions;
 
     var buffer = new StringBuffer(
         "Older versions of pub don't support ^ version constraints.\n"
         "Make sure your SDK constraint excludes those old versions:\n"
         "\n"
         "environment:\n"
         "  sdk: \"$newSdkConstraint\"\n"
@@ skipping 17 matching lines @@
     errors.add(buffer.toString().trim());
   }
 
   /// Returns the suggested version constraint for a dependency that was tested
   /// against [version].
   String _constraintForVersion(Version version) {
     if (_caretAllowed) return "^$version";
     return '">=$version <${version.nextBreaking}"';
   }
 }