fix(typeahead): Fix crash with contenteditable inputs

Currently the Typeahead module works well with `<input>`s, but throws an
error for HTML5 `contenteditable` inputs [1].

This patch fixes this by using the `innerText` property (the one used
for contenteditable inputs) when `value` is not available.

[1]: http://www.w3schools.com/tags/att_global_contenteditable.asp

components/typeahead/typeahead.directive.ts

@@ -92,7 +92,10 @@ export class TypeaheadDirective implements OnInit {
  }
  }
 
- if (e.target.value.trim().length >= this.typeaheadMinLength) {
+ // For `<input>`s, use the `value` property. For others that don't have a
+ // `value` (such as `<span contenteditable="true">`, use `innerText`.
+ const value = e.target.value !== undefined ? e.target.value : e.target.innerText;
+ if (value.trim().length >= this.typeaheadMinLength) {
  this.typeaheadLoading.emit(true);
  this.keyUpEventEmitter.emit(e.target.value);
  } else {