This change makes big_integer_arith_vm_test strong-mode compatible, and allows
dartdevc/dart2js to build and run big_integer tests where they can.
New issue 30170 filed to track DDC big integer handling.
BUG=
R=rnystrom@google.com
Review-Url: https://codereview.chromium.org/2978013002 .
Janice Collins