Tooling PRs

37508 build bundle with assemble

37642 Unit test for build.dart::GenSnapshot

37832 add –exit and –match-host-platform defaults to devicelab runner

37845 echo error messages to stderr

38560 refactor cocoapods validator to detect broken install

38576 flutter_tools/version: git log.showSignature=false

38632 Flutter Plugin Tool supports multi-platform plugin config

38654 [flutter_tool] Remove some async file io

38869 Store file hashes per build configuration.

38894 [flutter_tool] Move http request close under try-catch

38907 Throw error when hot reload enters bad state