Make Source.getDirectory synchronous.

lib/src/source.dart

Index: lib/src/source.dart
diff --git a/lib/src/source.dart b/lib/src/source.dart
index 85218d349791722d4552b8c3c3eafa9298dd3338..e49b17f737a4cfda1ef444b231f2bb912751e47f 100644
--- a/lib/src/source.dart
+++ b/lib/src/source.dart
@@ -107,10 +107,10 @@ abstract class Source {
 
   /// Returns the directory where this package can (or could) be found locally.
   ///
-  /// If the source is cached, this will be a path in the system cache. In that
-  /// case, this will return a directory even if the package has not been
-  /// installed into the cache yet.
-  Future<String> getDirectory(PackageId id);
+  /// If the source is cached, this will be a path in the system cache.
+  /// Depending on the source, this may throw an [ArgumentError] if [id] isn't
+  /// resolved using [resolveId].
+  String getDirectory(PackageId id);
 
   /// Gives the source a chance to interpret and validate the description for
   /// a package coming from this source.
@@ -182,6 +182,9 @@ abstract class Source {
   /// By default, this just returns [id].
   Future<PackageId> resolveId(PackageId id) => new Future.value(id);
 
+  /// Returns whether [id] is fully-resolved, according to [resolveId].
+  bool isResolved(PackageId id) => true;
+
   /// Returns the source's name.
   String toString() => name;
 }