Adds a package config file to the const finder test fixtures to unblock the null safety flag removal change.
Otherwise these fixtures get opted in to null safety and some constants are missing (presumably because they are invalid).
Related dart-lang/sdk#43872
Add copyright headers in a few files where they were missing.
Trim trailing blank comment line where present, for consistency with
other engine code.
Use the standard libtxt copyright header in one file where it differed
(extra (C) and comma compared to other files in libtxt).
This also amends tools/const_finder/test/const_finder_test.dart to look
for a const an additional four lines down to account for the copyright
header added to the test fixture.
