Phil Quitslund e44f6fe478 Bump Dart SDK to 1.20.0-dev.10.0. (#6212)
* Bump Dart SDK to `1.20.0-dev.10.0`.

`1.20.0-dev.10.0` corresponds to `1.20-RC0`.

* pubspec.lock cleanup

* check for pubspec.lock existence

* cleaned up conditional remove
2016-10-05 09:05:15 -07:00
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