Fixed handling of Dart MIME type

TBR=jacobr@google.com,alanknight@google.com

Review URL: https://codereview.chromium.org/1659793004 .

git-svn-id: https://src.chromium.org/blink/branches/dart/2454_1@202764 bbb929c8-8fbe-4397-9dbb-9b2b20218538
2 files changed