…e/Date.

Fixes #279 (initial report).

The internalization/externalization code only checks if the time
package is present and assumes time.Unix and time.Time are available if
so. Previously, it was possible for time package to be present, but
either time.Unix or time.Time were not (due to dead code elimination),
causing possible panics.

In time package itself, ensure time.Unix func and time.Time struct it
returns are always included (despite DCE) to prevent that from
happening.

Fixes #279 (additional findings).

When time package is not imported, it's still possible to internalize a
Date object (into an interface{}). Due to fallthrough being default,
this resulted in Function case running when internalizing Date with no
time package.

That makes no sense, and means that trying to externalize the Date
object back into JavaScript world would fail to produce the original
Date object (also, calling methods on it would obviously fail, since
it's not a real Date object).

For example, consider the following code that doesn't import time
package, it can now run and produce expected results:

	date := js.Global.Get("Date").New().Interface()
	js.Global.Get("console").Call("log", date)

Apply feedback from
#286 (comment).