Roll engine 834cd15010b2..31e5149a805b (3 commits) (#40549)

git@github.com:flutter/engine.git/compare/834cd15010b2...31e5149a805b

git log 834cd15010b2..31e5149a805b --no-merges --oneline
2019-09-16 iska.kaushik@gmail.com Revert "Revert "Roll src/third_party/dart a554c8be6b..d0052c1b31 (22 commits)" (#12290)" (flutter/engine#12291)
2019-09-16 iska.kaushik@gmail.com Revert "Add iOS platform view mutation XCUITests to the scenario app (#11652)" (flutter/engine#12292)
2019-09-15 ychris@google.com Add iOS platform view mutation XCUITests to the scenario app (flutter/engine#11652)


If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/flutter-engine-flutter-autoroll
Please CC jimgraham@google.com on the revert to ensure that a human
is aware of the problem.

To report a problem with the AutoRoller itself, please file a bug:
https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md