- Implement loading of .packages files from http.

runtime/bin/builtin.dart

Index: runtime/bin/builtin.dart
diff --git a/runtime/bin/builtin.dart b/runtime/bin/builtin.dart
index 5705c59a6aeea1d826a5b94fecf5daca8002f427..d678665b89917dd242a11cb98baf27972c612500 100644
--- a/runtime/bin/builtin.dart
+++ b/runtime/bin/builtin.dart
@@ -528,7 +528,7 @@ void _asyncLoadError(_LoadRequest req, _LoadError error, StackTrace stack) {
     Completer c = req._context;
     c.completeError(error, stack);
   } else {
-    Uri libraryUri = req._context;
+    String libraryUri = req._context;
     if (req._tag == _Dart_kImportTag) {
       // When importing a library, the libraryUri is the imported
       // uri.