Fix a missing url_request mojom import in fetch.dart.

Not exactly sure what I'm doing, but this makes the stocks example:

  `sky/tools/skydb start out/Debug/ sky/examples/stocks/main.sky`

work again, fixing the UrlRequest import issue:

[0603/143348:ERROR:dart_error.cc(20)]
'http://localhost:9999/packages/sky/framework/net/fetch.dart': malformed
type: line 32 pos 21: type 'UrlRequest' is not loaded
  var request = new UrlRequest()

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1148963007

sdk/lib/framework/net/fetch.dart

@@ -9,6 +9,7 @@ import 'dart:typed_data';
 import 'package:mojo/core.dart' as core;
 import 'package:mojom/mojo/network_service.mojom.dart';
 import 'package:mojom/mojo/url_loader.mojom.dart';
+import 'package:mojom/mojo/url_request.mojom.dart';
 
 class Response {
   ByteData body;