### Steps to Reproduce

- Create starter Stencil project
- Generate simple component
- Add attribute `@Watch("id")` with corresponding handler
- Add the component to the `index.html`
- Start stencil `npm start`
- Open page and inspect web component
- Change / add the id attribute on the web component
- id watcher did not trigger
- Add a random `@Prop() foo` to component file
- Change the web component `id` attribute again
- `id` watcher did trigger

Component code:
```jsx
import { Component, Host, Element, Watch, h, Prop } from '@stencil/core';

@Component({
  tag: 'test-input',
  styleUrl: 'test-input.css',
  shadow: true,
})
export class TestInput {
  @Element() hostElement: HTMLElement;

  private inputElement: HTMLInputElement;
  
  // Comment @Prop() to see @Watch("id") not working
  @Prop() fakeProp: string;

  @Watch("id")
  handleIdWatch(newId: HTMLElement["id"], currentId: HTMLElement["id"]) {
    console.log(`idWatcher: ${currentId} ---> ${newId}`);
    if (newId !== currentId) {
      // set new input id
      this.inputElement.id = newId;
    }
  }

  render() {
    return (
      <Host>
        <slot></slot>
        <input type="text" id={this.hostElement.id} ref={element => this.inputElement = element} />
      </Host>
    );
  }
}