Cmd-A doesn't select all text in the save dialog save-as field

[willfaught]

• VSCode Version: Version 1.10.2 (1.10.2), 8076a19
• OS Version: MacOS 10.12.3 (16D32)

Steps to Reproduce:

1. Create a new document.
2. Save it.
3. Move the cursor to the end of the default Save As text.
4. Press Cmd-A. Nothing is selected.

[bpasero]

This is the same as #19003, though in a different location.