Use single-property observer to deal better with if=undefined. Fixes #…

…1742

This takes advantage of the fact that single property observers are called for each change (including undefined), whereas multi-property observers wait for all properties to be defined.  Since the side-effect of the observe is to queue an async render, there is no real benefit from this being a multi-property observer.  #1946 is open to track a more general fix for this issue.

src/lib/template/dom-if.html

@@ -44,7 +44,8 @@
        */
       'if': {
         type: Boolean,
-        value: false
+        value: false,
+        observer: '_queueRender'
       },
 
       /**
@@ -56,7 +57,8 @@
        */
       restamp: {
         type: Boolean,
-        value: false
+        value: false,
+        observer: '_queueRender'
       }
 
     },
@@ -65,10 +67,6 @@
       Polymer.Templatizer
     ],
 
-    observers: [
-      '_queueRender(if, restamp)'
-    ],
-
     _queueRender: function() {
       this._debounceTemplate(this._render);
     },