Move pubspec caching into each source.

lib/src/command/cache_add.dart

Index: lib/src/command/cache_add.dart
diff --git a/lib/src/command/cache_add.dart b/lib/src/command/cache_add.dart
index 7958d27dfb51986488b4dd513821b131b4fe253c..d39fe96e12d691c778b8d4e324f867f4e1428668 100644
--- a/lib/src/command/cache_add.dart
+++ b/lib/src/command/cache_add.dart
@@ -59,8 +59,9 @@ class CacheAddCommand extends PubCommand {
     var source = cache.sources["hosted"];
 
     // TODO(rnystrom): Allow specifying the server.
-    var pubspecs = await source.getVersions(package, package);
-    var versions = pubspecs.map((pubspec) => pubspec.version)
+    var ids = await source.getVersions(
+        new PackageRef(package, 'hosted', package));
+    var versions = ids.map((id) => id.version)
         .where(constraint.allows).toList();
 
     if (versions.isEmpty) {