Handle cases where selection API doesn't apply

The selection API only applies to certain <input> types:

https://html.spec.whatwg.org/multipage/#do-not-apply

This commit ensures that we handle that correctly.

Some notes:

1. TextControl::set_dom_selection_direction now calls
   set_selection_range(), which means that setting selectionDirection will
   now fire a selection event, as it should per the spec.

2. There is a test for the firing of the select event in
   tests/wpt/web-platform-tests/html/semantics/forms/textfieldselection/select-event.html,
   however the test did not run due to this syntax error:

   (pid:26017) "ERROR:script::dom::bindings::error: Error at http://web-platform.test:8000/html/semantics/forms/textfieldselection/select-event.html:50:11 missing = in const declaration"

   This happens due to the us of the "for (const foo of ...)" construct.
   Per https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...of
   this should actually work, so it's somewhat unsatisfying to have to
   change the test.

4. If an <input>'s type is unset, it defaults to a text, and the
   selection API applies. Also, if an <input>'s type is set to an
   invalid value, it defaults to a text too. I've expanded the tests
   to account for this second case.

components/script/dom/htmltextareaelement.rs

@@ -8,6 +8,7 @@ use dom::bindings::codegen::Bindings::EventBinding::EventMethods;
 use dom::bindings::codegen::Bindings::HTMLTextAreaElementBinding;
 use dom::bindings::codegen::Bindings::HTMLTextAreaElementBinding::HTMLTextAreaElementMethods;
 use dom::bindings::codegen::Bindings::NodeBinding::NodeMethods;
+use dom::bindings::error::ErrorResult;
 use dom::bindings::inheritance::Castable;
 use dom::bindings::root::{DomRoot, LayoutDom, MutNullableDom};
 use dom::bindings::str::DOMString;
@@ -145,6 +146,10 @@ impl TextControl for HTMLTextAreaElement {
     fn textinput(&self) -> &DomRefCell<TextInput<ScriptToConstellationChan>> {
         &self.textinput
     }
+
+    fn selection_api_applies(&self) -> bool {
+        true
+    }
 }
 
 impl HTMLTextAreaElementMethods for HTMLTextAreaElement {
@@ -260,38 +265,38 @@ impl HTMLTextAreaElementMethods for HTMLTextAreaElement {
     }
 
     // https://html.spec.whatwg.org/multipage/#dom-textarea/input-selectionstart
-    fn SelectionStart(&self) -> u32 {
-        self.dom_selection_start()
+    fn GetSelectionStart(&self) -> Option<u32> {
+        self.get_dom_selection_start()
     }
 
     // https://html.spec.whatwg.org/multipage/#dom-textarea/input-selectionstart
-    fn SetSelectionStart(&self, start: u32) {
-        self.set_dom_selection_start(start);
+    fn SetSelectionStart(&self, start: Option<u32>) -> ErrorResult {
+        self.set_dom_selection_start(start)
     }
 
     // https://html.spec.whatwg.org/multipage/#dom-textarea/input-selectionend
-    fn SelectionEnd(&self) -> u32 {
-        self.dom_selection_end()
+    fn GetSelectionEnd(&self) -> Option<u32> {
+        self.get_dom_selection_end()
     }
 
     // https://html.spec.whatwg.org/multipage/#dom-textarea/input-selectionend
-    fn SetSelectionEnd(&self, end: u32) {
-        self.set_dom_selection_end(end);
+    fn SetSelectionEnd(&self, end: Option<u32>) -> ErrorResult {
+        self.set_dom_selection_end(end)
     }
 
     // https://html.spec.whatwg.org/multipage/#dom-textarea/input-selectiondirection
-    fn SelectionDirection(&self) -> DOMString {
-        self.dom_selection_direction()
+    fn GetSelectionDirection(&self) -> Option<DOMString> {
+        self.get_dom_selection_direction()
     }
 
     // https://html.spec.whatwg.org/multipage/#dom-textarea/input-selectiondirection
-    fn SetSelectionDirection(&self, direction: DOMString) {
-        self.set_dom_selection_direction(direction);
+    fn SetSelectionDirection(&self, direction: Option<DOMString>) -> ErrorResult {
+        self.set_dom_selection_direction(direction)
     }
 
     // https://html.spec.whatwg.org/multipage/#dom-textarea/input-setselectionrange
-    fn SetSelectionRange(&self, start: u32, end: u32, direction: Option<DOMString>) {
-        self.set_dom_selection_range(start, end, direction);
+    fn SetSelectionRange(&self, start: u32, end: u32, direction: Option<DOMString>) -> ErrorResult {
+        self.set_dom_selection_range(start, end, direction)
     }
 }