Improve the detection lockfile freshness.

lib/src/solver/version_solver.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.
 
 library pub.solver.version_solver;
 
 import 'dart:async';
 import "dart:convert";
 
 import 'package:pub_semver/pub_semver.dart';
@@ skipping 57 matching lines @@
   /// it was successful.
   final SolveFailure error;
 
   /// The number of solutions that were attempted before either finding a
   /// successful solution or exhausting all options.
   ///
   /// In other words, one more than the number of times it had to backtrack
   /// because it found an invalid solution.
   final int attemptedSolutions;
 
+  /// The [LockFile] representing the packages selected by this version
+  /// resolution.
+  LockFile get lockFile {
+    var sdkConstraint = new VersionConstraint.intersection(
+        pubspecs.values.map((pubspec) => pubspec.environment.sdkVersion));
+    return new LockFile(packages, _sources, sdkConstraint: sdkConstraint);
+  }
+
   final SourceRegistry _sources;
   final Package _root;
   final LockFile _previousLockFile;
 
   /// Returns the names of all packages that were changed.
   ///
   /// This includes packages that were added or removed.
   Set<String> get changedPackages {
     if (packages == null) return null;
 
@@ skipping 338 matching lines @@
 
   DependencyNotFoundException(String package, this._innerException,
       Iterable<Dependency> dependencies)
       : super(package, dependencies);
 
   /// The failure isn't because of the version of description of the package,
   /// it's the package itself that can't be found, so just show the name and no
   /// descriptive details.
   String _describeDependency(PackageDep dep) => "";
 }