ui.Paragraph.getLineBoundary gives an incorrect response when passed TextAffinity.downstream at a line wrap.

Example

Imagine we're rendering the string "123456789" and our field is very narrow and so the "9" wraps:

|12345678|
|9       |

There are now two TextPositions between the 8 and the 9, one on the first line and one on the second line:

First line: TextPosition(offset: 8, affinity: TextAffinity.upstream)
Second line: TextPosition(offset: 8, affinity: TextAffinity.downstream)

The problem is that getLineBoundary will return a TextRange of 0-8 regardless of which of the above two TextPositions are passed to it. It doesn't use the TextAffinity, only the index, which you can see in the code here.