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

Committed: https://chromium.googlesource.com/external/mojo/+/db73217e1ca565e3c9b208643154b879caa6d96f

[ppi, 2015-06-03 12:42:21]

ppi@chromium.org changed reviewers:
+ abarth@chromium.org, eseidel@chromium.org

[ppi, 2015-06-03 12:42:42]

Hi Adam or Eric,

ptal.

[eseidel, 2015-06-03 14:50:30]

lgtm

Dart doesn't re-export symbols from other libaries. So if url_request recently
split out from url_loader, then yeah this is needed.

[ppi, 2015-06-03 14:56:27]

Committed patchset #1 (id:1) manually as
db73217e1ca565e3c9b208643154b879caa6d96f (presubmit successful).