URL: URLSearchParams's size

For whatwg/url#734.

url/urlsearchparams-size.any.js

@@ -0,0 +1,34 @@
+test(() => {
+  const params = new URLSearchParams("a=1&b=2&a=3");
+  assert_equals(params.size, 3);
+
+  params.delete("a");
+  assert_equals(params.size, 1);
+}, "URLSearchParams's size and deletion");
+
+test(() => {
+  const params = new URLSearchParams("a=1&b=2&a=3");
+  assert_equals(params.size, 3);
+
+  params.append("b", "4");
+  assert_equals(params.size, 4);
+}, "URLSearchParams's size and addition");
+
+test(() => {
+  const url = new URL("http://localhost/query?a=1&b=2&a=3");
+  assert_equals(url.searchParams.size, 3);
+
+  params.delete("a");
+  assert_equals(url.searchParams.size, 1);
+
+  params.append("b", 4);
+  assert_equals(url.searchParams.size, 2);
+}, "URLSearchParams's size when obtained from a URL");
+
+test(() => {
+  const url = new URL("http://localhost/query?a=1&b=2&a=3");
+  assert_equals(url.searchParams.size, 3);
+
+  url.search = "?";
+  assert_equals(url.searchParams.size, 0);
+}, "URLSearchParams's size when obtained from a URL and using .search");