Default constructors for instantiable interfaces

[DartBot]

This issue was originally filed by @seaneagan

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Instantiable interfaces (those with a default class) should get a default constructor when they do not declare any, just like classes. For example, the following should be allowed:

interface Foo default _Foo {}

class _Foo implements Foo {}

main() => new Foo();

[sethladd]

Added Area-Language, Triaged labels.

[DartBot]

This comment was originally written by ross.m....@gmail.com

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I had a TODO in my code to report this as well - it feels strange having to explicitly declare the constructor in the interface in this situation.

[anders-sandholm]

Added this to the M1 milestone.

[gbracha]

Set owner to @gbracha.
Removed Type-Defect label.
Added Type-Enhancement, Accepted labels.

[dgrove]

Issue #3410 has been merged into this issue.

[gbracha]

Another problem that is rendered moot by the unification of interfaces and classes.

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Added WontFix label.