Move pubspec caching into each source.

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.
 
 library pub.validator.dependency;
 
 import 'dart:async';
 
 import 'package:pub_semver/pub_semver.dart';
 
@@ skipping 49 matching lines @@
 
     if (caretDeps.isNotEmpty && !_caretAllowed) {
       _errorAboutCaretConstraints(caretDeps);
     }
   }
 
   /// Warn that dependencies should use the hosted source.
   Future _warnAboutSource(PackageDep dep) async {
     var versions;
     try {
-      var pubspecs = await entrypoint.cache.sources['hosted']
-          .getVersions(dep.name, dep.name);
-      versions = pubspecs.map((pubspec) => pubspec.version).toList();
+      var ref = new PackageRef(dep.name, 'hosted', dep.name);
+      var ids = await entrypoint.cache.sources['hosted'].getVersions(ref);
+      versions = ids.map((id) => id.version).toList();
     } catch (error) {
       versions = [];
     }
 
     var constraint;
     var primary = Version.primary(versions);
     if (primary != null) {
       constraint = _constraintForVersion(primary);
     } else {
       constraint = dep.constraint.toString();
@@ skipping 125 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}"';
   }
 }