Make it possible to inherit from any constructable host object

This CL makes it possible for authors to extend any host object (e.g., DOM
objects) and to use those objects in all the usual places they can be used in
the API.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/936193005

engine/src/flutter/tests/dom/inherit-from-text-expected.txt

@@ -0,0 +1,6 @@
+CONSOLE: unittest-suite-wait-for-done
+CONSOLE: PASS: should be able to insert in DOM
+CONSOLE: 
+CONSOLE: All 1 tests passed.
+CONSOLE: unittest-suite-success
+DONE

engine/src/flutter/tests/dom/inherit-from-text.sky

@@ -0,0 +1,31 @@
+<sky>
+<script>
+import "../resources/third_party/unittest/unittest.dart";
+import "../resources/unit.dart";
+
+import "dart:sky";
+
+class CustomText extends Text {
+  CustomText() : super("awesome");
+
+  bool get isCustom => true;
+}
+
+void main() {
+  initUnit();
+
+  test("should be able to insert in DOM", () {
+    var child = new CustomText();
+    expect(child.isCustom, isTrue);
+    expect(child.parentNode, isNull);
+    expect(child.data, equals("awesome"));
+
+    var parent = document.createElement("div");
+    parent.appendChild(child);
+    expect(child.parentNode, equals(parent));
+    expect(parent.firstChild, equals(child));
+    expect(parent.firstChild.isCustom, isTrue);
+  });
+}
+</script>
+</sky>