Actually make computed readOnly:true. Add more tests.

test/unit/bind-elements.html

@@ -248,6 +248,10 @@
       computedvalue="[[boundcomputedvalue]]"
       computednotifyingvalue="[[boundcomputednotifyingvalue]]">
     </x-basic>
+    <x-basic id="basic3"
+      notifyingvalue="{{computedValue}}"
+      computedvalue="{{value}}">
+    </x-basic>
   </template>
   <script>
     Polymer({
@@ -259,6 +263,17 @@
         'boundcomputednotifyingvalueChanged(boundcomputednotifyingvalue)',
         'boundreadonlyvalueChanged(boundreadonlyvalue)'
       ],
+      properties: {
+        a: {
+          value: 10
+        },
+        b: {
+          value: 20
+        },
+        computedValue: {
+          computed: 'computeComputedValue(a, b)'
+        }
+      },
       created: function() {
         this.observerCounts = {
           boundvalueChanged: 0,
@@ -268,6 +283,9 @@
           boundreadonlyvalueChanged: 0
         };
       },
+      computeComputedValue: function(a, b) {
+        return a + b;
+      },
       clearObserverCounts: function() {
         for (var i in this.observerCounts) {
           this.observerCounts[i] = 0;