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) {
+      if (resolvedUri == null) {
+        throw new ArgumentError.value(uri, "uri", "Unknown package");
+      }
+      return resolvedUri;
+    });
   }
-  return Uri.base.resolveUri(uri);
+  return new Future<Uri>.value(Uri.base.resolveUri(uri));
 }