File-argument not opened when starting editor.

[andersjohnsen]

When I run

./DartEditor my_file.dart

I expect DartEditor to open the file my_file.dart, but I just get the default empty workspace. Would be very neat!

[stevemessick]

Removed Type-Defect label.
Added Type-Enhancement, Triaged labels.

[clayberg]

Added this to the Later milestone.

[stevemessick]

Added Duplicate label.
Marked as being merged into #344.

[clayberg]

Removed this from the Later milestone.
Added this to the M1 milestone.
Marked as being merged into #dart:344.