Pub oauth2 test fails with RangeError

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="96" height="96"hspace="10"> Issue by nex3
Originally opened as dart-lang/sdk#7015

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Logs: http://build.chromium.org/p/client.dart/builders/pub-win/builds/814/steps/pub%20tests%20failures/logs/stdio

[DartBot]

<img src="https://avatars.githubusercontent.com/u/46275?v=3" align="left" width="48" height="48"hspace="10"> Comment by munificent

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On Mac, at least, this was because we were reading the header file before it had finished getting written. We're reading it later now, so this should be fixed.

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Added Fixed label.