[dart:convert] Utf8Decoder lack Iterator support

[AgainPsychoX]

I would love use Utf8Decoder from dart:convert (and other similar too) like this:

Iterator<int> myIterator; // ...
utf8.decode(myIterator, /*length:*/ 17);

The same goes for just Iterables (note: those can be copied down toList, so these are so urgent).