The RB.TextCommentRowSelector has been cleaned up, modernized, and

ported to ES6. All the method documentation is now aligned with our

documentation standards.
There were two cases of event handlers (for mousedown and mouseup)

which were sometimes returning False to prevent the default event

from ocurring. However, this behaviour is not actually supported; the

DOM events for mousedown and mouseup do not have preventDefault in

their prototypes and so the default behaviour cannot be prevented.
The _copySelectionToClipboard has been modernized to not do repeated

string concatentation. Instead it builds up an array of strings and

joins them with the detected platform's newline character.
In addition, a function that _copySelectionToClipboard was defining

has been pulled out into the prototype to make it clear that it does not

close over any variables in the method.

Ran JS tests.