Support `MessageChannel._usePoly` rather than flipping a hardcoded flag

[hjdivad]

This will make the tests nicer too; we can always use poly in tests.

if( !root.MessageChannel || root.MessageChannel._usePoly)

[hjdivad]

btw why do we create self = this when we pass in the global object as root?

[hjdivad]

cc @Cyril-sf