Fix widget.dart to work with the latest web_ui library. Specifically, query selectors need to use "…

lib/components/carousel.dart

Index: lib/components/carousel.dart
diff --git a/lib/components/carousel.dart b/lib/components/carousel.dart
index c07a12b0595d19a796f4266f2bb7955705c2a396..bf31358013831090cc84e96ec47758b32050be95 100644
--- a/lib/components/carousel.dart
+++ b/lib/components/carousel.dart
@@ -25,20 +25,21 @@ class Carousel extends WebComponent {
 
   Future<bool> previous() => _moveDelta(false);
 
-  SwapComponent get _swap => this.query('x-carousel > .carousel > x-swap').xtag;
+  SwapComponent get _swap =>
+      this.query('[is=x-carousel] > .carousel > [is=x-swap]').xtag;
 
   Future<bool> _moveDelta(bool doNext) {
     final swap = _swap;
     assert(swap != null);
-    if(swap.items.length == 0) {
-      return new Future.immediate(false);
+    if (swap.items.length == 0) {
+      return new Future.value(false);
     }
 
     assert(doNext != null);
     final delta = doNext ? 1 : -1;
 
     ShowHideEffect showEffect, hideEffect;
-    if(doNext) {
+    if (doNext) {
       showEffect = _fromTheRight;
       hideEffect = _fromTheLeft;
     } else {
@@ -50,6 +51,7 @@ class Carousel extends WebComponent {
 
     final newIndex = (activeIndex + delta) % _swap.items.length;
 
-    return _swap.showItemAtIndex(newIndex, effect: showEffect, hideEffect: hideEffect, duration: _duration);
+    return _swap.showItemAtIndex(newIndex, effect: showEffect,
+        hideEffect: hideEffect, duration: _duration);
   }
 }