[polymer] import all elements

pkg/polymer/lib/elements/web-animations-js/test/testcases/test-reversed-player-active-phase.html

+<style>
+#target {
+    position: absolute;
+    width: 10px;
+    height: 10px;
+    background: black;
+}
+</style>
+<script src="../bootstrap.js"></script>
+<div id="target"></div>
+<script>
+timing_test(function() {
+    var player = target.animate([{left: '0px'}, {left: '100px'}], {duration: 2, fill: 'forwards'}).player;
+
+    at(1, function() {
+        assert_styles(target, {left: '50px'});
+        player.currentTime = 3;
+        player.playbackRate = -1;
+        assert_styles(target, {left: '100px'});
+    }, 'Player reversing');
+
+    at(2, function() {}, 'Tick');
+
+    at(3, function() {
+        assert_styles(target, {left: '50px'});
+    }, 'Reversed player entering its active phase should take effect');
+});
+</script>