When compiling TypeScript targeting ES5, if --downlevelIteration isn't
used, then arrays of tuples won't be considered assignable to iterables
of tuples, resulting in type errors when doing common things like
Map([['key', 'value']]):

microsoft/TypeScript#32761

--downlevelIteration hurts runtime performance of for..of loops on
arrays, but a type definition-only workaround is to explicitly include
arrays or arrays of tuples in parameter types or function overloads.

This apparently used to be masked by #1611, but became
a problem when #1626 was merged.