Create a TypeDefiningElement interface.
Create a TypeDefiningElement interface.

This interface is common to ClassElement, FunctionTypeAliasElement,
and the element for the pseudo-type "dynamic".  This should allow us
to avoid some unnecessary casting in the dev_compiler (see
https://codereview.chromium.org/1160223006/#msg11).

R=brianwilkerson@google.com

Review URL: https://codereview.chromium.org//1165213002

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