dropBackWhile abstract method
Drops characters from the end of the range while they satisfy test
.
Iterates the characters of the current range backwards from the end
and removes all the iterated characters until one is
reached for which test
returns false
.
If on such character is found, all characters are removed,
which gives the same effect as collapseToStart.
Implementation
void dropBackWhile(bool Function(String) test);