Add browser-compatible version.

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 {
+  if (uri.scheme == "package") {
+    return Isolate.resolvePackageUri(uri);
+  }
+  return Uri.base.resolveUri(uri);
+}