Set FLUTTER_ROOT to an absolute path
Set FLUTTER_ROOT to an absolute path

This implementation was taken from the Dart SDK shell scripts.

BUG=https://github.com/flutter/flutter/issues/2795

tvolkert/flutter,cbracken/flutter,cbracken/flutter,cbracken/flutter,tvolkert/flutter,flutter/flutter,flutter/flutter,aghassemi/flutter,aghassemi/flutter,tvolkert/flutter,turnidge/flutter,turnidge/flutter,turnidge/flutter,Hixie/flutter,aghassemi/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,Hixie/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,tvolkert/flutter,tvolkert/flutter,flutter/flutter,tvolkert/flutter,tvolkert/flutter,turnidge/flutter,tvolkert/flutter,cbracken/flutter,turnidge/flutter,Hixie/flutter,Hixie/flutter,aghassemi/flutter,flutter/flutter,Hixie/flutter,cbracken/flutter,flutter/flutter,Hixie/flutter,cbracken/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,cbracken/flutter,Hixie/flutter,cbracken/flutter,flutter/flutter,cbracken/flutter,flutter/flutter