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

Locate the byContentVal decorator and inspect how it observes content when the decorated value is on the node itself rather than a child node. Reproduce the ButtonModel example, add the promised test case, and confirm that changing the node contents updates label.

Written by the indexing model from the issue text.