Cache packageName to base Uri mapping.

This makes analysis with using incremental analysis cache 30% faster.
Without this change getBase() takes 40% (!) of total analysis time.
With this change - just 0.79% of total time.

Although the remaining 17% of PackagesBase.resolve() make my cry.
Why URI manipulations are SO SLOW?!

R=brianwilkerson@google.com, pquitslund@google.com, kevmoo@google.com
BUG=

Review URL: https://codereview.chromium.org/2041103005 .

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

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