Make BaseRequest.contentType use null rather than -1 as a flag value.

BUG=
R=rnystrom@google.com

Review URL: https://codereview.chromium.org//196423017

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@33781 260f80e4-7a28-3924-810f-c04153c831b5
1 file changed
tree: 223ae1320917368d31f6c20dc2f852500e43f925
  1. lib/
  2. test/
  3. LICENSE
  4. pubspec.yaml