Throw if trying to set fs.currentDirectory in tests (#22037)
Throw if trying to set fs.currentDirectory in tests (#22037)

* Block setting of fs.currentDirectory in tests

This isn't perfect, it only covers tests using testUsingContext, but that is the huge majority of tests.

* Tweak error message.

tvolkert/flutter,Hixie/flutter,Hixie/flutter,flutter/flutter,cbracken/flutter,flutter/flutter,tvolkert/flutter,flutter/flutter,flutter/flutter,Hixie/flutter,cbracken/flutter,tvolkert/flutter,flutter/flutter,tvolkert/flutter,Hixie/flutter,tvolkert/flutter,Hixie/flutter,cbracken/flutter,cbracken/flutter,Hixie/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,Hixie/flutter,Hixie/flutter,cbracken/flutter,cbracken/flutter,tvolkert/flutter,tvolkert/flutter,tvolkert/flutter,flutter/flutter,cbracken/flutter,Hixie/flutter,cbracken/flutter,flutter/flutter,flutter/flutter,cbracken/flutter,cbracken/flutter,tvolkert/flutter,flutter/flutter