Fix a missing url_request mojom import in fetch.dart.
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

afandria/mojo,afandria/mojo,afandria/mojo,chinmaygarde/mojo,jianglu/mojo,chinmaygarde/mojo,jianglu/mojo,chinmaygarde/mojo,jianglu/mojo,jianglu/mojo,afandria/mojo,jianglu/mojo,jianglu/mojo,chinmaygarde/mojo,chinmaygarde/mojo,chinmaygarde/mojo,afandria/mojo,chinmaygarde/mojo,chinmaygarde/mojo,afandria/mojo,afandria/mojo,jianglu/mojo,jianglu/mojo,chinmaygarde/mojo,afandria/mojo