Feature Request: Support ?? operator, and perhaps also ??!

[tfortes]

In Dart, you often have to type:

  String aTextValue = getText();
  String y = (aTextValue != null) ? aTextValue : 'Default';

...but the null-coalescing operator ?? simplifies this by returning LHS if LHS is non-null and RHS otherwise.

  String y = aTextValue ?? 'Default';

Going one step further, we could also add the ??! operator that returns null if LHS is null, and RHS otherwise.

Element child = getChild();
Element parent = child ??! child.parent;

[The operator names are debatable of course]

[kevmoo]

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

[kevmoo]

Added Duplicate label.
Marked as being merged into #1236.