* Update project.pbxproj files to say Flutter rather than Chromium
Also, the templates now have an empty organization so that we don't cause people to give their apps a Flutter copyright.
* Update the copyright notice checker to require a standard notice on all files
* Update copyrights on Dart files. (This was a mechanical commit.)
* Fix weird license headers on Dart files that deviate from our conventions; relicense Shrine.
Some were already marked "The Flutter Authors", not clear why. Their
dates have been normalized. Some were missing the blank line after the
license. Some were randomly different in trivial ways for no apparent
reason (e.g. missing the trailing period).
* Clean up the copyrights in non-Dart files. (Manual edits.)
Also, make sure templates don't have copyrights.
* Fix some more ORGANIZATIONNAMEs
* MultiTapGestureRecognizer previously would assert if there was no
competition.
* GestureArenaTeam would always select the first recongizer as the
winner even if a later recognizer actually accepted the pointer
sequence.
* debugPrintStack would fail a type check if maxFrames was non-null.
* FractionalOffset.lerp would throw a null-pointer exception if its
second argument was null.
Also, add a number of tests for previously untested lines of code.
