Don't set expression attributes before they're bound.

sky/tests/framework/templates.sky

 <sky>
 <import src="/sky/tests//resources/chai.sky" />
 <import src="/sky/tests/resources/mocha.sky" />
+<import src="/sky/framework/sky-element/sky-element.sky" as="SkyElement" />
 <import src="/sky/tests/resources/test-element.sky" as="TestElement" />
 
 <div id="sandbox"></div>
 
 <test-element id="parser-element" checked="true" size="10" name="foo bar" />
 
+<sky-element name="test-element-parent">
+  <template>
+    <test-element size="{{ size }}" />
+  </template>
+  <script>
+    (class extends SkyElement {
+      created() {
+        this.size = 10;
+      }
+    }).register();
+  </script>
+</sky-element>
+
+<test-element-parent id="parent" />
+
 <script>
 describe("SkyElement", function() {
   var element;
   var sandbox = document.getElementById("sandbox");
 
   beforeEach(function() {
     element = new TestElement();
   });
   afterEach(function() {
     element.remove();
@@ skipping 160 matching lines @@
 
   it("should connect host event handlers", function() {
     sandbox.appendChild(element);
     element.dispatchEvent(new CustomEvent("wrong-event"));
     assert.isNull(element.lastEvent);
     var event = new CustomEvent("host-event");
     element.dispatchEvent(event);
     assert.equal(element.lastEvent, event);
   });
 
+  it("should not call attributeChanged for binding expression values", function() {
+    var parent = document.getElementById('parent');
+    var element = parent.shadowRoot.querySelector('test-element');
+    assert.deepEqual(element.changes, [
+      { name: 'size', oldValue: 0, newValue: 10 },
+    ]);
+  });
+
   it("should call shadowRootReady after creating the template instance", function() {
     assert.equal(element.shadowRootReadyCount, 0);
     sandbox.appendChild(element);
     assert.equal(element.shadowRootReadyCount, 1);
     element.remove();
     sandbox.appendChild(element);
     assert.equal(element.shadowRootReadyCount, 1);
   });
 });
 </script>
 </sky>