Workaround deliberate overflowing negation in serialize::json.

Author: pnkfelix

src/libserialize/json.rs

@@ -1540,7 +1540,8 @@ impl<T: Iterator<Item=char>> Parser<T> {
             F64Value(res)
         } else {
             if neg {
-                let res = -(res as i64);
+                // FIXME (#24420) use wrapping_neg instead after it lands.
+                let res = !(res as i64).wrapping_add(1);
 
                 // Make sure we didn't underflow.
                 if res > 0 {