Add data-URI support class to dart:core (next to Uri).

runtime/bin/builtin.dart

Index: runtime/bin/builtin.dart
diff --git a/runtime/bin/builtin.dart b/runtime/bin/builtin.dart
index d6d945eb48e4b4e23fc73235e8fb2ef47dad985b..5ae31aea9b3d26fa4369c39b122f59628d3b96fb 100644
--- a/runtime/bin/builtin.dart
+++ b/runtime/bin/builtin.dart
@@ -44,9 +44,9 @@ _getCurrentDirectoryPath() native "Builtin_GetCurrentDirectory";
 Uri _uriBase() {
   // We are not using Dircetory.current here to limit the dependency
   // on dart:io. This code is the same as:
-  //   return new Uri.file(Directory.current.path + "/");
-  var result = _getCurrentDirectoryPath();
-  return new Uri.file("$result/");
+  //   return new Uri.directory(Directory.current.path);
+  var path = _getCurrentDirectoryPath();
+  return new Uri.directory(path);
 }