check if dynamic Type has a given instance

[DartBot]

This issue was originally filed by @seaneagan

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"x is int" is nice, but sometimes instead of "int" we have some dynamic Type object. Would be nice to be able to instance of checks in this case as well, something like:

reflectClass(someType).hasInstance(x);

or

reflect(x).isA(someType);

[madsager]

Added Area-Language, Triaged labels.

[gbracha]

You can do this with mirrors. We don't want to support this in the base language because of performance constraints.

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Set owner to @gbracha.
Added WontFix label.

[DartBot]

This comment was originally written by @zoechi

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but it is strongly discouraged to use mirrors for client code which is intended to be built to JS and this is a very common requirement.

[gbracha]

How common it is debatable. I'd love to have first class types be much more powerful, but we will not undermine the performance of core language constructs (which are really widely used).