Create package for HTML imports

pkg/html_import/README.md

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+# HTML Imports polyfill

[Jennifer Messerly, 2013/07/25 19:35:11]

link the spec?
https://dvcs.w3.org/hg/webcomponents/raw-file/tip/spec/imports/index.html

[Siggi Cherem (dart-lang), 2013/07/25 20:48:40]

Done.

+
+HTML Imports are a way to include and reuse HTML documents in other HTML documents. As `<script>` tags let authors include external Javascript in their pages, imports let authors load full HTML resources.  In particular, imports let authors include [Custom Element](https://github.com/Polymer/CustomElements) definitions from external URLs.

[Jennifer Messerly, 2013/07/25 19:34:27]

long line :)

[Siggi Cherem (dart-lang), 2013/07/25 20:48:40]

Done. I copied it over from their file =)

+
+
+## Getting Started
+
+Include the `html-imports.js` or `html-imports.min.js` (minified) file in your project.
+
+    <script src="packages/html_import/html-imports.js"></script>

[Jennifer Messerly, 2013/07/25 19:34:27]

this should be _?

[Siggi Cherem (dart-lang), 2013/07/25 20:48:40]

Their file is actually '-', should I rename it?

If so, maybe I should match the name of the package? html_import.js (or
html_import.debug.js) and html_import.min.js

Thoughts?

[Jennifer Messerly, 2013/07/25 20:51:08]

yeah, I would rename it to match pkg name. After chatting w/ some folks I
changed Shadow DOM to match Dart pkg naming guidelines: shadow_dom

IMHO, making changes to conform to expected pkg/asset naming conventions in Dart
is totally okay...

+
+`html-imports.js` is the debug loader and uses `document.write` to load additional modules. 
+Use the minified version (`html-imports.min.js`) if you need to load the file dynamically.
+
+## Basic usage
+
+For HTML imports use the `import` relation on a standard `<link>` tag, for example:
+
+    <link rel="import" href="import-file.html">
+
+## Polyfill details
+
+You can read more about how the polyfill is implemented in JavaScript here:
+<https://github.com/Polymer/HTMLImports/tree/master#polyfill-details>
+
+## Getting the source code
+
+This package is built from:
+<https://github.com/Polymer/HTMLImports/tree/master>
+
+You'll need [node.js](http://nodejs.org) to rebuild the JS file. Use `npm install` to
+get dependencies and `grunt` to build.