The fault (for fear of simply rewording what the Closure Compiler error says) is that IE8 and below cannot parse object literals and array literals which have a trailing comma.
var obj = {
a: 1,
b: 2, // trailing comma is bad!
};
... vs...
var obj = {
a: 1,
b: 2 // no trailing comma!
};
The fix is to remove the trailing comma.