Change fromString constructor to parse static method

on HeaderValue and ContentType.

R=ajohnsen@google.com
BUG=

Review URL: https://codereview.chromium.org//14914002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/http@22406 260f80e4-7a28-3924-810f-c04153c831b5
3 files changed
tree: e930ac4fe3acc4e792cc0f3be02e5e96a04bae67
  1. lib/
  2. test/
  3. pubspec.yaml