byContentVal does not update in cases where there are no children

### Expected Behaviour

A simple model like this should update if the contents of the node changes:
```
class ButtonModel extends DOMModel {
@byContentVal() label;
}
```

### Actual Behaviour

The initial contents of the DOM node is read, but the model does not respond to changes

### Reproduce Scenario (including but not limited to)

Any use of the `byContentVal` decorator that isn't in a child node

#### Steps to Reproduce

I will submit a PR that includes a test case

#### Platform and Version

Chrome 72, MacOS

#### Sample Code that illustrates the problem

I will submit a PR that includes a test case

#### Logs taken while reproducing problem

定位 byContentVal 装饰器，并检查当被装饰的值位于节点本身而不是子节点上时，它如何观察内容。复现 ButtonModel 示例，添加承诺的测试用例，并确认更改节点内容会更新 label。

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