Cache pubspecs when HostedSource.getVersions is called.

sdk/lib/_internal/pub/lib/src/source.dart

Index: sdk/lib/_internal/pub/lib/src/source.dart
diff --git a/sdk/lib/_internal/pub/lib/src/source.dart b/sdk/lib/_internal/pub/lib/src/source.dart
index 7affb689ab1454f1c7aeaedcdaaee2584d818a09..85218d349791722d4552b8c3c3eafa9298dd3338 100644
--- a/sdk/lib/_internal/pub/lib/src/source.dart
+++ b/sdk/lib/_internal/pub/lib/src/source.dart
@@ -57,7 +57,7 @@ abstract class Source {
     this._systemCache = systemCache;
   }
 
-  /// Get the list of all versions that exist for the package described by
+  /// Get the pubspecs of all versions that exist for the package described by
   /// [description].
   ///
   /// [name] is the expected name of the package.
@@ -69,9 +69,9 @@ abstract class Source {
   ///
   /// By default, this assumes that each description has a single version and
   /// uses [describe] to get that version.
-  Future<List<Version>> getVersions(String name, description) {
+  Future<List<Pubspec>> getVersions(String name, description) async {
     var id = new PackageId(name, this.name, Version.none, description);
-    return describe(id).then((pubspec) => [pubspec.version]);
+    return [await describe(id)];
   }
 
   /// Loads the (possibly remote) pubspec for the package version identified by