Fix hanging of flutter run when running on more than one simulator. (#23932)
Fix hanging of flutter run when running on more than one simulator. (#23932)

This stops reading from pipe-to-file log after the build is completed. Attempting to read from pipe-to-file keeps open blocked because nobody is going to write into that pipe.
tvolkert/flutter,cbracken/flutter,flutter/flutter,cbracken/flutter,flutter/flutter,cbracken/flutter,flutter/flutter,flutter/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,flutter/flutter,Hixie/flutter,tvolkert/flutter,flutter/flutter,tvolkert/flutter,flutter/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,tvolkert/flutter,tvolkert/flutter,Hixie/flutter,tvolkert/flutter,cbracken/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,cbracken/flutter,tvolkert/flutter,flutter/flutter,tvolkert/flutter,Hixie/flutter,cbracken/flutter,flutter/flutter,tvolkert/flutter,tvolkert/flutter,Hixie/flutter,Hixie/flutter,cbracken/flutter