Port remaining Sky tests to the new world

sky/tests/dom/getChildElements.dart

+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 doc = new Document();
+    var parent = doc.createElement('parent');
+    var child1 = doc.createElement('child1');
+    var child2 = doc.createElement('child1');
+    var grandchild = doc.createElement('grandchild');
+
+    doc.appendChild(parent);
+    parent.appendChild(child1);
+    parent.appendChild(child2);
+    child1.appendChild(grandchild);
+
+    var children = parent.getChildElements();
+    expect(children.length, equals(2));
+    expect(children[0], equals(child1));
+    expect(children[1], equals(child2));
+  });
+}