POSIX woes in t7810.87: dash bash or bash dash?

GNU bash, Version 4.2.10(1)-release (x86_64-redhat-linux-gnu):

echo "a\"b\n\"c"
a"b\n"c

dash-0.5.6:

echo "a\"b\n\"c"
a"b
"c

t7810.87 breaks with dash because of this. Escaping the \n does not make
the test work with both either. So I'd like to know which one's right
and (independently) how to make it work for both...

Cheers,
Michael