oauth2: Add text/javascript as a possible response content-type when validate token

[DartBot]

Originally opened as dart-lang/sdk#17067

This issue was originally filed by orig...@gmail.com

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https://codereview.chromium.org/177093006

Some api providers like dropbox (for example /token endpoint) set content-type response to text/javascript so _validate call fail.

Unhandled exception:
FormatException: Invalid OAuth response for "https://api.dropbox.com/1/oauth2/token": content-type was "text/javascript", expected "application/json".

Response is {"access_token": "TOKEN", "token_type": "bearer", "uid": "UID"}

[DartBot]

<img src="https://avatars.githubusercontent.com/u/5479?v=3" align="left" width="48" height="48"hspace="10"> Comment by sethladd

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cc @nex3.
Added Area-Pkg, Pkg-OAuth, Triaged labels.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="48" height="48"hspace="10"> Comment by nex3

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It's frustrating that Dropbox doesn't follow the spec, but I suppose there's no benefit to being persnickety about it in the client.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="48" height="48"hspace="10"> Comment by nex3

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Fixed by dart-lang/sdk@a5cc4f6.

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Added Fixed label.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="48" height="48"hspace="10"> Comment by nex3

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Removed Pkg-OAuth label.
Added Pkg-OAuth2 label.