67152 [null-safety] pass experiments to builders (cla: yes, framework, tool)

67165 [flutter_tools] update build rules to depend on subset of package_config contents (cla: yes, tool)

67171 [null-safety] add integration tests for sound null safety modes, add support for sound null safety in dart2js (a: null-safety, cla: yes, team, tool, waiting for tree to go green)

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)

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