Update division migrator for Dart 2.0.0 semantics

[nex3]

(Note: this shouldn't land on the main branch until Dart 2.0.0 is released.)

In Dart 2.0.0, / will be treated as a list separator and list.slash() will be deprecated. This means that the current migration target for the division migration will need a further migration to replace list.slash() references with literal lists.

See sass/sass#2565