This change replaces usages of Polymer global variable with imported
functions. The bundled-polymer.js are still imported, but only in entry
points: gr-app.js and embed/gr-diff.js as well as in test setup (to
still have Polymer available on `window` to plugins and embed usages).
Change-Id: I0afec9272c7bb752db7da23ca6364f64b230fe2b