Miscellaneous async code fix fixes

[uniqueiniquity]

This is generally cleanup, so not wholly necessary for TS 3.1.

• Remove unnecessary call to concat on a ReadonlyArray to convert to a mutable array
• Clarify meaning of when an argName is set as undefined
• Add test demonstrating when undefined is passed to catch
• Narrow annotated type
• Rename function to point out that a default value is produced

[amcasey]