covariant method
Assert that JavaScript's x
is the proper type represented by this
Type.
Implementation
@override
bool covariant(dynamic x) {
return (x is Map &&
_fields.every((entry) {
var k = entry.key;
var t = entry.value;
if (!x.containsKey(k)) {
throw "Record is missing key '$k'.";
}
return t.covariant(x[k]);
}));
}