Cache packageName to base Uri mapping.

lib/src/packages_io_impl.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// Implementations of [Packages] that can only be used in server based
 /// applications.
 library package_config.packages_io_impl;
 
 import "dart:collection" show UnmodifiableMapView;
 import "dart:io" show Directory;
 import "package:path/path.dart" as path;
 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
         .listSync()
         .where((e) => e is Directory)
         .map((e) => path.basename(e.path));
   }
 
   Iterable<String> get packages => _listPackageNames();
 
   Map<String, Uri> asMap() {
     var result = <String, Uri>{};
     for (var packageName in _listPackageNames()) {
       result[packageName] = getBase(packageName);
     }
     return new UnmodifiableMapView<String, Uri>(result);
   }
 }