Use pub_semver package in pub.

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Review URL: https://codereview.chromium.org//602253002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@40849 260f80e4-7a28-3924-810f-c04153c831b5
diff --git a/lib/src/pubspec.dart b/lib/src/pubspec.dart
index fad03b2..55b2df7 100644
--- a/lib/src/pubspec.dart
+++ b/lib/src/pubspec.dart
@@ -5,6 +5,7 @@
 library pub.pubspec;
 
 import 'package:path/path.dart' as path;
+import 'package:pub_semver/pub_semver.dart';
 import 'package:source_span/source_span.dart';
 import 'package:yaml/yaml.dart';
 
@@ -14,7 +15,6 @@
 import 'package.dart';
 import 'source_registry.dart';
 import 'utils.dart';
-import 'version.dart';
 
 /// The parsed contents of a pubspec file.
 ///