Add requestSubmit() polyfill by default (#439)

So we can rely on requestSubmit everywhere and know it'll work on Safari.

src/polyfills/form-request-submit-polyfill.js

@@ -0,0 +1,51 @@
+/**
+ * The MIT License (MIT)
+ * 
+ * Copyright (c) 2019 Javan Makhmali
+ * 
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ * 
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ * 
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+(function(prototype) {
+  if (typeof prototype.requestSubmit == "function") return
+
+  prototype.requestSubmit = function(submitter) {
+    if (submitter) {
+      validateSubmitter(submitter, this)
+      submitter.click()
+    } else {
+      submitter = document.createElement("input")
+      submitter.type = "submit"
+      submitter.hidden = true
+      this.appendChild(submitter)
+      submitter.click()
+      this.removeChild(submitter)
+    }
+  }
+
+  function validateSubmitter(submitter, form) {
+    submitter instanceof HTMLElement || raise(TypeError, "parameter 1 is not of type 'HTMLElement'")
+    submitter.type == "submit" || raise(TypeError, "The specified element is not a submit button")
+    submitter.form == form || raise(DOMException, "The specified element is not owned by this form element", "NotFoundError")
+  }
+
+  function raise(errorConstructor, message, name) {
+    throw new errorConstructor("Failed to execute 'requestSubmit' on 'HTMLFormElement': " + message + ".", name)
+  }
+})(HTMLFormElement.prototype);

src/polyfills/index.ts

@@ -1,2 +1,3 @@
 import "./custom-elements-native-shim"
+import "./form-request-submit-polyfill"
 import "./submit-event"