Cast operator (`.as<...>`) in expression language prevents errors reporting
Take the following KSY:
```yaml
meta:
  id: test_type
instances:
  me:
    value: unknown.as<u1>
```
When `.as<u1>` part is missing, the error is reported, as it should:
```
test_type: /instances/me/value:
	error: unable to access 'unknown' in test_type context
```
... but when it is present, that code is generated:
```js
function TestType(_io, _parent, _root) {
  this._io = _io;
  this._parent = _parent;
  this._root = _root || this;
  this._debug = {};

}
TestType.prototype._read = function() {
}
Object.defineProperty(TestType.prototype, 'me', {
  get: function() {
    if (this._m_me !== undefined)
      return this._m_me;
    this._debug._m_me = {  };
    this._m_me = this.unknown;
    return this._m_me;
  }
});
```
I checked several fields where expressions is allowed and the same behavior is observed in each case.