Don't send all barback assets as text/html.
Don't send all barback assets as text/html.

R=rnystrom@google.com
BUG=

Review URL: https://codereview.chromium.org//134563004

git-svn-id: a962b240025cf0577aa172e20f49ccb159ba6be1@31707 260f80e4-7a28-3924-810f-c04153c831b5

dart-lang/pub,computmaxer/pub,dart-lang/pub,maxwellpeterson-wf/pub,cjkao/dart-pub