diff --git a/lib/src/packages_io_impl.dart b/lib/src/packages_io_impl.dart
index db39bdb..0e94746 100644
--- a/lib/src/packages_io_impl.dart
+++ b/lib/src/packages_io_impl.dart
@@ -14,10 +14,15 @@ import "packages_impl.dart";
 /// A [Packages] implementation based on a local directory.
 class FilePackagesDirectoryPackages extends PackagesBase {
   final Directory _packageDir;
+  final Map<String, Uri> _packageToBaseUriMap = <String, Uri>{};
+
   FilePackagesDirectoryPackages(this._packageDir);
 
-  Uri getBase(String packageName) =>
-      new Uri.file(path.join(_packageDir.path, packageName, '.'));
+  Uri getBase(String packageName) {
+    return _packageToBaseUriMap.putIfAbsent(packageName, () {
+      return new Uri.file(path.join(_packageDir.path, packageName, '.'));
+    });
+  }
 
   Iterable<String> _listPackageNames() {
     return _packageDir
diff --git a/pubspec.yaml b/pubspec.yaml
index 80aea6d..3d9bd45 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: package_config
-version: 0.1.4
+version: 0.1.5
 description: Support for working with Package Resolution config files.
 author: Dart Team <misc@dartlang.org>
 homepage: https://github.com/dart-lang/package_config