Fix Isolate.spawnUri on Windows.

runtime/lib/isolate_patch.dart

 // Copyright (c) 2012, 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.
 
 patch class ReceivePort {
   /* patch */ factory ReceivePort() = _ReceivePortImpl;
 
   /* patch */ factory ReceivePort.fromRawReceivePort(RawReceivePort rawPort) =
       _ReceivePortImpl.fromRawReceivePort;
 }
@@ skipping 195 matching lines @@
       rethrow;
     };
     return completer.future;
   }
 
   /* patch */ static Future<Isolate> spawnUri(
       Uri uri, List<String> args, var message) {
     Completer completer = new Completer<Isolate>.sync();
     try {
       // The VM will invoke [_startIsolate] and not `main`.
-      SendPort controlPort = _spawnUri(uri.path);
+      SendPort controlPort = _spawnUri(uri.toString());
       RawReceivePort readyPort = new RawReceivePort();
       controlPort.send([readyPort.sendPort, args, message]);
       readyPort.handler = (readyMessage) {
         assert(readyMessage == 'started');
         readyPort.close();
         completer.complete(new Isolate._fromControlPort(controlPort));
       };
     } catch(e, st) {
       // TODO(floitsch): we want errors to go into the returned future.
       rethrow;
     };
     return completer.future;
   }
 
   static final ReceivePort _port =
       new ReceivePort.fromRawReceivePort(_getPortInternal());
 
   static SendPort _spawnFunction(Function topLevelFunction)
       native "isolate_spawnFunction";
 
   static SendPort _spawnUri(String uri) native "isolate_spawnUri";
 }