update shadowdom polyfill to latest

pkg/shadow_dom/tool/README.md

Index: pkg/shadow_dom/tool/README.md
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+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
+```