title: How to observe property changes with LitElement
body: <p>I don't know how to observe property changes in LitElement. </p>
  
  <p>I've been trying these methods, but I couldn't get these to work:</p>
  
  <pre><code>  static get properties() {
      return {
        temp1:String,
        temp2: {
         type:String,
         observer: '_temp2Changed'
          }
    };
  
    temp1Changed(newValue, oldValue){
      console.log("temp1 changed")
    }
    _temp2Changed(newValue, oldValue){
      console.log("temp2 changed")
    }
  </code></pre>