67231 [flutter_tools] do not use IOSink for writing cache responses (cla: yes, tool)

67234 [flutter_tools] remove globals from FlutterValidator, add documentation and move tests to new file (cla: yes, tool)

67235 Roll Engine from a24c7c13925e to 167f45cdd893 (6 revisions) (cla: yes, waiting for tree to go green)

67237 [flutter_tools] add more docs to cocoapods, move to globals (cla: yes, tool)

67240 [flutter_tools] remove globals from IntelliJ validator, refactor tests to remove dependency on JAR (cla: yes, tool)

67242 [flutter_tools] Simplify plugin test cases and expand coverage of AndroidPlugin (cla: yes, tool)