Remove redundant dynamic_casts.

[scott-snyder]

Also get an exception if the cast fails, rather than a null pointer dereference.

BEGINRELEASENOTES

• Remove redundant uses of dynamic_cast.
  ENDRELEASENOTES

[jmcarcell]

Note that this changes the behaviour in case of nullptr since dynamic casting to a reference will throw std::bad_cast. We'll see tomorrow if this has any impact.