Address a number of open issues.

lib/src/resolve.dart

 // Copyright (c) 2016, 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.
 
 import "dart:async" show Future;
 import "dart:isolate" show Isolate;
 
 /// Helper function for resolving to a non-relative, non-package URI.
-Future<Uri> resolveUri(Uri uri) async {
+Future<Uri> resolveUri(Uri uri) {
   if (uri.scheme == "package") {
-    return Isolate.resolvePackageUri(uri);
+    return Isolate.resolvePackageUri(uri).then((resolvedUri) =>
+      resolvedUri ?? new ArgumentError.value(uri, "uri", "Unknown package"));

[floitsch, 2017/02/01 12:17:59]

That looks wrong.
I'm guessing you wanted to throw the argument error.
I prefer not to use ?? for that.

[Lasse Reichstein Nielsen, 2017/02/01 12:44:36]

True, should have had a throw.
Rewriting to the full if-statement.

   }
-  return Uri.base.resolveUri(uri);
+  return new Future<Uri>.value(Uri.base.resolveUri(uri));
 }