64-bit integers in the language tour

[anders-sandholm]

We should update https://www.dartlang.org/guides/language/language-tour#numbers to account for the new Dart 2 semantics with 64-bit integers.

[anders-sandholm]

This is already covered in #507

[kwalrath]

Fixed: https://dartlang-org-dev.firebaseapp.com/guides/language/language-tour#numbers