introduce web_components pkg for consolidated polyfills

pkg/shadow_dom/tool/README.md

-This folder contains the logic for building the shadow_dom package's
-concatenated and minified JS files.
-
-## Prerequisites
-
-Install nodejs and npm. On Debian based systems this is typically:
-
-```bash
-     sudo apt-get install nodejs
-     sudo apt-get install npm
-```
-- Install grunt-cli:
-
-```bash
-    npm install -g grunt-cli
-```
-
-See the Grunt [getting started](http://gruntjs.com/getting-started) page
-for more information.
-
-## Building
-
-Run shadow_dom/tool/build.sh (from any directory):
-
-```bash
-    ./build.sh
-```
-
-## (optional) How to integrate Polymer upstream changes
-
-One time setup:
-
-```bash
-    # Note: this requires commit access to dart-lang/ShadowDOM.
-    # You can use your own fork instead if you like.
-    # Just use that URL here and edit build.sh to pull from there.
-    git clone -b shadowdom_patches https://github.com/dart-lang/ShadowDOM.git
-    cd ShadowDOM
-    git remote add upstream https://github.com/Polymer/ShadowDOM.git
-```
-
-You can merge upstream changes by doing:
-
-```bash
-    # Check that we are in shadowdom_patches branch and don't have
-    # any pending changes.
-    git status
-
-    git fetch upstream
-    git merge upstream/master
-    git push origin shadowdom_patches
-```