Make ResolveUri return the correct target_uri for relative base uris.

runtime/vm/uri.cc

 // 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.
 
 #include "vm/uri.h"
 
 #include "vm/zone.h"
 
 namespace dart {
 
@@ skipping 518 matching lines @@
 
   } else {
     // Relative path.  We need to merge the base path and the ref path.
 
     if (base.scheme == NULL && base.host == NULL && base.path[0] != '/') {
       // The dart:core Uri class handles resolving a relative uri
       // against a second relative uri specially, in a way not
       // described in the RFC.  We do not need to support this for
       // library resolution.  If we need to implement this later, we
       // can.
+      *target_uri = NULL;
       return false;
     }
 
     target.scheme = base.scheme;
     target.userinfo = base.userinfo;
     target.host = base.host;
     target.port = base.port;
     target.path = RemoveDotSegments(MergePaths(base.path, ref.path));
     target.query = ref.query;
     target.fragment = ref.fragment;
     *target_uri = BuildUri(target);
     return true;
   }
 }
 
 }  // namespace dart