Update Polymer components.

third_party/polymer/components/iron-flex-layout/iron-flex-layout.html

Index: third_party/polymer/components/iron-flex-layout/iron-flex-layout.html
diff --git a/third_party/polymer/components/iron-flex-layout/iron-flex-layout.html b/third_party/polymer/components/iron-flex-layout/iron-flex-layout.html
index a14a872b38d252365cc00585e1e5bf02e77a1f7e..a69f46a3c1f3f30ce7857376c07a863e351c51d1 100644
--- a/third_party/polymer/components/iron-flex-layout/iron-flex-layout.html
+++ b/third_party/polymer/components/iron-flex-layout/iron-flex-layout.html
@@ -20,14 +20,27 @@ The layout class stylesheet provides a simple set of class-based flexbox rules,
 let you specify layout properties directly in markup. You must include this file
 in every element that needs to use them.
 
-  Sample use:
-
-      <link rel="import" href="../iron-flex-layout/iron-flex-layout-classes.html">
-      <style is="custom-style" include="iron-flex iron-flex-alignment"></style>
-
-      <div class="layout horizontal layout-start">
-        <div>cross axis start alignment</div>
-      </div>
+    Sample use:
+
+    ```
+    <custom-element-demo>
+      <template>
+        <script src="../webcomponentsjs/webcomponents-lite.js"></script>
+        <next-code-block></next-code-block>
+      </template>
+    </custom-element-demo>
+    ```
+
+    ```html
+    <link rel="import" href="iron-flex-layout-classes.html">
+    <style is="custom-style" include="iron-flex iron-flex-alignment"></style>
+    <style>
+      .test { width: 100px; }
+    </style>
+    <div class="layout horizontal center-center">
+      <div class="test">horizontal layout center alignment</div>
+    </div>
+    ```
 
 2. [Custom CSS mixins](https://github.com/PolymerElements/iron-flex-layout/blob/master/iron-flex-layout.html).
 The mixin stylesheet includes custom CSS mixins that can be applied inside a CSS rule using the `@apply` function.
@@ -105,7 +118,7 @@ A complete [guide](https://elements.polymer-project.org/guides/flex-layout) to `
       -webkit-flex-wrap: wrap;
       flex-wrap: wrap;
     };
-    
+
     --layout-no-wrap: {
       -ms-flex-wrap: nowrap;
       -webkit-flex-wrap: nowrap;