[third_party] add polymer 1.2.3

polymer_1.2.3/bower_components/iron-flex-layout/README.md

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+<!---
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+This README is automatically generated from the comments in these files:
+iron-flex-layout.html
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+Edit those files, and our readme bot will duplicate them over here!
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+-->
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+[![Build Status](https://travis-ci.org/PolymerElements/iron-flex-layout.svg?branch=master)](https://travis-ci.org/PolymerElements/iron-flex-layout)
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+_[Demo and API Docs](https://elements.polymer-project.org/elements/iron-flex-layout)_
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+
+##&lt;iron-flex-layout&gt;
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+The `<iron-flex-layout>` component provides simple ways to use [CSS flexible box layout](https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Flexible_boxes), also known as flexbox. This component provides two different ways to use flexbox:
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+1. [Layout classes](https://github.com/PolymerElements/iron-flex-layout/tree/master/classes). The layout class stylesheet provides a simple set of class-based flexbox rules. Layout classes let you specify layout properties directly in markup.
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+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.
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+A complete [guide](https://elements.polymer-project.org/guides/flex-layout) to `<iron-flex-layout>` is available.
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+