Depend on hosted rather than sdk for unittest

This avoids conflicts in things depending on this
Update code to recent Dart

No warnings or errors on 16761
Update documentation to point to correct site

Signed-off-by: Daniel Thomas <drt24@cam.ac.uk>
Truncating division is the same as division for integers
Add library big_integer declaration to lib/src/BigInteger/big_integer.dart so that unit tests run
Florian Schneider's optimisation patch

Add patch from https://github.com/financeCoding/dart-bignum/issues/8
this is supposed to be an optimisation to make it go faster, it does
so by not using a Map but instead an auto-growing array to do storage
of the internal data structure.

It was also necessary to fix a type error this revealed.

I did not include the additions of parentheses which the patch on github
introduced as they seemed to add unnecessary noise to the patch (and I
don't see what they are for).
Merge pull request #9 from ucam-cl-dtg/master

Fix to recent dart