Ensure that VM service extension handlers are run on outer event loop. (#17812)
Ensure that VM service extension handlers are run on outer event loop. (#17812)

Fixes https://github.com/flutter/flutter/issues/17597
tvolkert/flutter,Hixie/flutter,cbracken/flutter,cbracken/flutter,flutter/flutter,tvolkert/flutter,tvolkert/flutter,flutter/flutter,Hixie/flutter,tvolkert/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,flutter/flutter,cbracken/flutter,cbracken/flutter,cbracken/flutter,cbracken/flutter,cbracken/flutter,tvolkert/flutter,flutter/flutter,flutter/flutter,flutter/flutter,flutter/flutter,flutter/flutter,Hixie/flutter,tvolkert/flutter,Hixie/flutter,cbracken/flutter,cbracken/flutter,Hixie/flutter,Hixie/flutter,tvolkert/flutter,cbracken/flutter,Hixie/flutter,Hixie/flutter,tvolkert/flutter,Hixie/flutter,tvolkert/flutter,flutter/flutter