Make CompilerOutput immutable (#16942)
Make CompilerOutput immutable (#16942)

CompilerOutput values should never be mutated. Also includes minor
formatting change to the compile() method signature in line with the
style guide.

Minor housekeeping while I'm touching other bits of nearby code.
cbracken/flutter,tvolkert/flutter,flutter/flutter,tvolkert/flutter,tvolkert/flutter,flutter/flutter,Hixie/flutter,flutter/flutter,Hixie/flutter,Hixie/flutter,flutter/flutter,flutter/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,tvolkert/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,cbracken/flutter,cbracken/flutter,cbracken/flutter,cbracken/flutter,Hixie/flutter,cbracken/flutter,flutter/flutter,tvolkert/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,tvolkert/flutter,Hixie/flutter,cbracken/flutter,flutter/flutter,Hixie/flutter