Add support to the version solver for sources that only have a single version per package.

utils/pub/root_source.dart

Index: utils/pub/root_source.dart
diff --git a/utils/pub/root_source.dart b/utils/pub/root_source.dart
index 1e8d4f400bf1225da1f9057afbfc968f63f82a24..b1081589376a97e6addac2edb330456d30af14ea 100644
--- a/utils/pub/root_source.dart
+++ b/utils/pub/root_source.dart
@@ -20,10 +20,6 @@ class RootSource extends Source {
 
   RootSource(this.package);
 
-  Future<List<Version>> getVersions(description) {
-    return Future<List<Version>>.immediate([package.version]);
-  }
-
   Future<Pubspec> describe(PackageId id) {
     return Future<Pubspec>.immediate(package.pubspec);
   }