Language: Allow interface members to be declared abstract

[DartBot]

This issue was originally filed by kresten...@gmail.com

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Sometimes you want to convert an abstract class to an interface and vice versa. When doing this, you have to go through the code and add/remove "abstract" in front of all member declarations.

It would be harmless to permit the members of an interface to be declared abstract; and that would make it easier to convert such a class.

[DartBot]

This comment was originally written by drfibonacci@google.com

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

[gbracha]

Set owner to @gbracha.
Added Accepted label.

[anders-sandholm]

The added convenience does not compensate for the increased confusion.
 

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Added WontFix label.