70126 [null-safety] implement autodetection for the web (cla: yes, tool)

70132 Revert “[flutter_tools] restore pub caching functionality” (cla: yes, tool)

70133 [flutter_tools] restore pub caching functionality (cla: yes, tool)

70136 [flutter_tools] always run pub with prebuilt applications on drive (cla: yes, tool)

70144 Revert “[flutter_tools] restore pub caching functionality” (cla: yes, tool)

70146 [flutter_tools] always use dart to run test script. (cla: yes, tool)

70175 Revert “[flutter_tools] always use dart to run test script.” (cla: yes, tool)

70180 [flutter_tools] reland: cache pub invocations (cla: yes, tool)