Add a basic test for Mimic tree movement

packages/unit/test/widget/build_utils.dart

@@ -40,6 +40,15 @@ class WidgetTester {
   TestApp _app;
   RenderView _renderView;
 
+  void walkWidgets(WidgetTreeWalker walker) {
+    void walk(Widget widget) {
+      walker(widget);
+      widget.walkChildren(walk);
+    }
+
+    _app.walkChildren(walk);
+  }
+
   void pumpFrame(WidgetBuilder builder) {
     _app.builder = builder;
     Component.flushBuild();

packages/unit/test/widget/mimic_test.dart

+import 'package:sky/widgets.dart';
+import 'package:test/test.dart';
+
+import 'build_utils.dart';
+
+void main() {
+  test('Mimic can track tree movements', () {
+    GlobalKey globalKey = new GlobalKey();
+    WidgetTester tester = new WidgetTester();
+
+    tester.pumpFrame(() {
+      return new Flex([
+        new Mimicable(
+          key: globalKey,
+          child: new Container(
+            key: new Key.stringify('inner'),
+            height: 10.0,
+            width: 10.0
+          )
+        )
+      ]);
+    });
+
+    bool mimicReady = false;
+
+    tester.pumpFrame(() {
+      return new Flex([
+        new Mimicable(
+          key: globalKey,
+          child: new Container(
+            height: 10.0,
+            width: 10.0
+          )
+        ),
+        new SizedBox(
+          height: 10.0,
+          width: 10.0,
+          child: new Mimic(
+            original: globalKey,
+            onMimicReady: () {
+              mimicReady = true;
+            }
+          )
+        )
+      ]);
+    });
+
+    expect(mimicReady, isTrue);
+
+    tester.pumpFrame(() {
+      return new Flex([
+        new Container(
+          key: new Key.stringify('outer'),
+          height: 10.0,
+          width: 10.0,
+          child: new Mimicable(
+            key: globalKey,
+            child: new Container(
+              key: new Key.stringify('inner'),
+              height: 10.0,
+              width: 10.0
+            )
+          )
+        ),
+        new SizedBox(
+          height: 10.0,
+          width: 10.0,
+          child: new Mimic(original: globalKey)
+        )
+      ]);
+    });
+
+  });
+}