added (get)lastVisibleRow to editor.dart and virtual_renderer.dart

@rmsmith - We needed to add (get)lastVisibleRow.

I began to implement tests for this, but it looks like a mock virtual renderer would have to be added to the tests (lastVisibleRow always reports 0).  What do you suggest?  Have you attempted this?  There is a mock renderer in the ace.js tests, but doesn't look like it's been used in ace.dart's tests.