Mutation Observers provide a way to react to changes in the DOM. This is needed on IE versions 9 and 10, see http://caniuse.com/mutationobserver.
Include the polyfill in your HTML <head>:
<script src="packages/mutation_observer/mutation_observer.js"></script>
You can also use a minified version for deployment:
<script src="packages/mutation_observer/mutation_observer.min.js"></script>
The source for this package is at: https://code.google.com/p/dart/source/browse/branches/bleeding_edge/dart/pkg/mutation_observer/
The original source of the JavaScript code is at: https://github.com/Polymer/MutationObservers/tree/master
The minified version is produced with:
uglifyjs mutation_observer.js -o mutation_observer.min.js
See https://github.com/mishoo/UglifyJS2 for usage of UglifyJS.