Please merge 6bc1cdb5b149036f6610605921e28215fabe3d84 into dev channel

[alexmarkov]

commit(s) to merge: 6bc1cdb

merge instructions: clean merge

reason: this CL introduces --experimental_unsafe_mode_use_at_your_own_risk VM option which could be used as a temporary workaround for performance regressions in Dart tools after they were switched to Dart 2.

/cc @dgrove @kevmoo @mit-mit @whesse @athomas @a-siva

[whesse]

Merged in 2.0.0-dev.69.2