js-sys: Add extends to Math

Part of #670

crates/js-sys/src/lib.rs

@@ -1203,6 +1203,7 @@ extern {
 #[wasm_bindgen]
 extern "C" {
     #[derive(Clone, Debug)]
+    #[wasm_bindgen(extends = Object)]
     pub type Math;
 
     /// The Math.abs() function returns the absolute value of a number, that is

crates/js-sys/tests/wasm/Math.rs

@@ -1,9 +1,22 @@
 use std::f64::consts::PI;
 use std::f64::{NEG_INFINITY, NAN};
 
+use wasm_bindgen::{JsCast, prelude::*};
 use wasm_bindgen_test::*;
 use js_sys::*;
 
+#[wasm_bindgen_test]
+fn math_extends() {
+    #[wasm_bindgen]
+    extern {
+        #[wasm_bindgen(js_name = Math)]
+        static math: Math;
+    }
+
+    assert!(math.is_instance_of::<Object>());
+    let _: &Object = math.as_ref();
+}
+
 macro_rules! assert_eq {
     ($a:expr, $b:expr) => ({
         let (a, b) = (&$a, &$b);