feat(ngModelController): hold listener during text composition

When composing text in CJKV, intermediate buffer for unfinished text should not
be updating the bound scope variables.

Closes angular#4684

src/ng/directive/input.js

@@ -393,8 +393,21 @@ var inputType = {
 
 
 function textInputType(scope, element, attr, ctrl, $sniffer, $browser) {
+  // In composition mode, users are still inputing intermediate text buffer,
+  // hold the listener until composition is done.
+  // More about composition events: https://developer.mozilla.org/en-US/docs/Web/API/CompositionEvent
+  var composing = false;
+
+  element.on('compositionstart', function() {
+    composing = true;
+  });
+
+  element.on('compositionend', function() {
+    composing = false;
+  });
 
   var listener = function() {
+    if (composing) return;
     var value = element.val();
 
     // By default we will trim the value