dartc allows default instantiation of an interface

[DartBot]

This issue was originally filed by mmendez@google.com

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Implicit default constructors should only be added to classes.

interface IA {}
main() {
  new IA();
}

However, if an interface declares a factory that is default instantiable, then it maybe desirable to synthesize a default constructor for the interface as well.

[DartBot]

This comment was originally written by mmendez@google.com

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Fixed in http://code.google.com/p/dart/source/detail?r=2470.

Instantiating an interface with a constructor but no factory will be addressed separately.

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Added Fixed label.