We want this so they can be efficiently stored in vectors using move semantics. 

Attempting to add "noexcept" to the underlying flat_tree move constructor results in a Clang error on Mac and iOS:

In file included from ../../base/containers/flat_tree_unittest.cc:5:
../../base/containers/flat_tree.h:308:21: error: exception specification of 'vector' uses itself
    underlying_type body_;
                    ^
../../base/containers/flat_tree.h:308:21: note: in instantiation of exception specification for 'vector' requested here
../../buildtools/third_party/libc++/trunk/include/type_traits:3507:41: note: in instantiation of template class 'std::__1::is_constructible<std::__1::allocator<base::MoveOnlyInt>, std::__1::allocator<base::MoveOnlyInt> &&>' requested here
    : __libcpp_is_nothrow_constructible<is_constructible<_Tp, _Args...>::value, is_reference<_Tp>::value, _Tp, _Args...>
                                        ^
../../buildtools/third_party/libc++/trunk/include/type_traits:3658:14: note: in instantiation of template class 'std::__1::is_nothrow_constructible<std::__1::allocator<base::MoveOnlyInt>, std::__1::allocator<base::MoveOnlyInt> &&>' requested here
    : public is_nothrow_constructible<_Tp, typename add_rvalue_reference<_Tp>::type>
             ^
../../buildtools/third_party/libc++/trunk/include/vector:559:20: note: in instantiation of template class 'std::__1::is_nothrow_move_constructible<std::__1::allocator<base::MoveOnlyInt> >' requested here
        _NOEXCEPT_(is_nothrow_move_constructible<allocator_type>::value);
                   ^
../../base/containers/flat_tree.h:308:21: note: in instantiation of exception specification for 'vector' requested here
    underlying_type body_;
                    ^
../../base/containers/flat_tree_unittest.cc:307:16: note: in instantiation of member function 'base::internal::flat_tree<base::MoveOnlyInt, base::MoveOnlyInt, base::internal::GetKeyFromValueIdentity<base::MoveOnlyInt>, std::__1::less<base::MoveOnlyInt> >::flat_tree' requested here
  MoveOnlyTree moved(std::move(original));
               ^
../../base/containers/flat_tree.h:338:53: note: implicit move constructor for 'base::internal::flat_tree<base::MoveOnlyInt, base::MoveOnlyInt, base::internal::GetKeyFromValueIdentity<base::MoveOnlyInt>, std::__1::less<base::MoveOnlyInt> >' first required here
flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::flat_tree(