Allow automatic save in Dart Editor

[DartBot]

This issue was originally filed by @ahmetaa

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In Dart Editor, after you change the code and hit run, system asks you if changes should be saved on a modal dialog. Similarly for errors to appear, user needs to hit save button. That is very clumsy IMHO.

It would be nice if Editor automatically saves the changes on frame deactivation (As seen in IntelliJ, current file is saved after user moves the focus out of the current edit area). Granted that editor has local history support but I think it is doable. This mechanism also prevents the problem where user does not save for a long time and after a crash edited code is lost.

In my opinion this mechanism is superior as I never hit save button for years and never missed it or lost a line of code.

[kasperl]

Added Area-Editor, Triaged labels.

[stevemessick]

Removed Type-Defect label.
Added Type-Enhancement label.

[keertip]

Added Duplicate label.
Marked as being merged into #695.

[clayberg]

Added this to the M2 milestone.
Marked as being merged into #2960.