BREAKING CHANGE:
Previously, it was possible for an action passed to $watch
to be a string, interpreted as an angular expresison. This is no longer supported.
The action now has to be a function.
Passing an action to $watch is still optional.
Before:
```js
$scope.$watch('state', ' name="" ');
```
After:
```js
$scope.$watch('state', function () {
$scope.name = "";
});
```
Closes#8190
02c0ed27bc