Start adding support for lock files.

utils/pub/source_registry.dart

Index: utils/pub/source_registry.dart
diff --git a/utils/pub/source_registry.dart b/utils/pub/source_registry.dart
index e6080e39534ef3722c211f4be67e4b0140c26013..481db8a233a583a687ead8060de71d277dee2811 100644
--- a/utils/pub/source_registry.dart
+++ b/utils/pub/source_registry.dart
@@ -50,6 +50,11 @@ class SourceRegistry {
   }
 
   /**
+   * Returns `true` if there is a source named [name].
+   */
+  bool contains(String name) => _map.containsKey(name);
+
+  /**
    * Returns the source named [name]. Throws an error if no such source has been
    * registered. If [name] is null, returns the default source.
    */