Use pub_semver package in pub.

sdk/lib/_internal/pub/lib/src/solver/backtracking_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.
 
 /// A back-tracking depth-first solver.
 ///
 /// Attempts to find the best solution for a root package's transitive
 /// dependency graph, where a "solution" is a set of concrete package versions.
 /// A valid solution will select concrete versions for every package reached
 /// from the root package's dependency graph, and each of those packages will
@@ skipping 20 matching lines @@
 /// recent speculative version choice and try the next one. That becomes the
 /// new in-progress solution and it tries to proceed from there. It will keep
 /// doing this, traversing and then backtracking when it meets a failure until
 /// a valid solution has been found or until all possible options for all
 /// speculative choices have been exhausted.
 library pub.solver.backtracking_solver;
 
 import 'dart:async';
 import 'dart:collection' show Queue;
 
+import 'package:pub_semver/pub_semver.dart';
+
 import '../barback.dart' as barback;
 import '../exceptions.dart';
 import '../lock_file.dart';
 import '../log.dart' as log;
 import '../package.dart';
 import '../pubspec.dart';
 import '../sdk.dart' as sdk;
 import '../source_registry.dart';
 import '../source/unknown.dart';
 import '../utils.dart';
-import '../version.dart';
 import 'dependency_queue.dart';
 import 'version_queue.dart';
 import 'version_solver.dart';
 
 /// The top-level solver.
 ///
 /// Keeps track of the current potential solution, and the other possible
 /// versions for speculative package selections. Backtracks and advances to the
 /// next potential solution in the case of a failure.
 class BacktrackingSolver {
@@ skipping 712 matching lines @@
 ///
 /// Throws a [SolveFailure] if not.
 void _validateSdkConstraint(Pubspec pubspec) {
   if (pubspec.environment.sdkVersion.allows(sdk.version)) return;
 
   throw new BadSdkVersionException(pubspec.name,
       'Package ${pubspec.name} requires SDK version '
       '${pubspec.environment.sdkVersion} but the current SDK is '
       '${sdk.version}.');
 }