Add TestInternalizeExternalizeNull.
Add TestInternalizeExternalizeNull.

It's just like TestInternalizeExternalizeUndefined, but for null. It
helped me uncover a bug in internalization code during development, and
seems like a good idea to have it for better coverage.

bpowers/gopherjs,gopherjs/gopherjs,Archs/gopherjs,albrow/gopherjs,Yanjing123/gopherjs,gopherjs/gopherjs,bpowers/gopherjs,jargv/gopherjs,gopherjs/gopherjs,flimzy/gopherjs,govlas/gopherjs,albrow/gopherjs,albrow/gopherjs,jargv/gopherjs,govlas/gopherjs,jargv/gopherjs,flimzy/gopherjs,bpowers/gopherjs,Yanjing123/gopherjs,Archs/gopherjs,flimzy/gopherjs,Archs/gopherjs,Yanjing123/gopherjs,gopherjs/gopherjs,gopherjs/gopherjs,govlas/gopherjs