Look at this Plunker.
It looks like line 1 on index.html that reads:
<script src="http://www.polymer-project.org/1.0/samples/components/webcomponentsjs/webcomponents-lite.min.js"></script>
plus line 1 on parent-element.html that reads:
<link rel="import" href="http://www.polymer-project.org/1.0/samples/components/polymer/polymer.html">
are collectively importing all the Polymer 1.0 web elements.
Is that correct? Or are they only importing a subset?
Plunker doesn't automatically import all Polymer elements.
The first reference is the polyfill library (see http://webcomponents.org/polyfills/)
webcomponents.js is a set of polyfills built on top of the Web Components specifications. It makes it possible for developers to use these standards today across all modern browsers.
The second reference is the Polymer library itself.
I believe your answer to importing elements into Plunker has already been answered as well:
How do I import Polymer elements into code playgrounds like plunker (plnkr.co), jsBin and jsFiddle?