Fix getChildElements.

sky/tests/dom/getChildElements.sky

+<parent>
+  <child1>
+    <grandchild />
+  </child1>
+  <child2 />
+</parent>
+
+<script>
+import "../resources/third_party/unittest/unittest.dart";
+import "../resources/unit.dart";
+
+import "dart:sky";
+
+void main() {
+  initUnit();
+
+  test("getChildElements should only include immediate children", () {
+    var parent = document.querySelector('parent');
+    var children = parent.getChildElements();
+    expect(children.length, equals(2));
+    expect(children[0], equals(document.querySelector('child1')));
+    expect(children[1], equals(document.querySelector('child2')));
+  });
+}
+</script>
+</sky>