In both definitions of regionMatches in class String, I believe the line

        int plim = po + other.count;

should read

        int plim = other.offset + other.count;

Given that currently String objects do not use nonzero offsets, this error should
not affect running code in any observable way; but if String should ever be changed
to use nonzero offset values, then certian calls to regionMatches will behave incorrectly
by returning false when they should return true.