Lib: 2017/10

Since Polymer doesn't support expression directly, I create an express function as expression alternative.
Demo source
<html>
<head>
    <title>Express function for Polymer</title>
    <base href="https://cdn.rawgit.com/download/polymer-cdn/2.0.2/lib/">
    <link rel="import" href="polymer/polymer.html" />
</head>
<body>
    <h1>Expression</h1>
    <expressed-content count="15"></expressed-content>
 
    <dom-module id="expressed-content">
        <template>
            <template is="dom-repeat" items="[[sequences]]" as="item">
                <!-- This means: -->
                <!-- 1. sqrt(item) as $result -->
                <!-- 2. printText($result, item) as output -->
                <div>[[express(item, sqrt, printText, item)]]</div>
            </template>
        </template>
        <script>
            'use strict';
            class ExpressedContent extends Polymer.Element {
 
                static get is() { return 'expressed-content'; }
 
                static get properties() {
                    return {
                        count: {
                            type: Number,
                            value: 10,
                            observer: '_countChanged'
                        },
 
                        sequences: {
                            type: Array
                        }
                    }
                }
 
                _countChanged(n, o) {
                    n = Math.max(0, n * 1);
                    let sequences = [];
                    for (let i = 0; i < n; i++) {
                        sequences.push(i + 1);
                    }
                    this.sequences = sequences;
                }
 
                /**
                 * An express function instead of expression
                 */
                express() {
                    let args = [...arguments];
 
                    // first argument should be first parameter
                    let params = [args.shift()];
                    // second argument should be a function
                    let fn = args.shift();
 
                    // arguments before next function will be additional parameters
                    while (args[0] && typeof (args[0]) !== 'function') {
                        params.push(args.shift());
                    }
 
                    // execute and receive result
                    let nextMainParam = fn.call(this, ...params);
 
                    // continue express when args is not empty
                    return args.length === 0 ? nextMainParam : this.express(nextMainParam, ...args);
                }
 
                sqrt(n) {
                    return n * n;
                }
 
                printText(n1, n2) {
                    return `${n2} x ${n2} = ${n1}`;
                }
 
            }
            customElements.define(ExpressedContent.is, ExpressedContent);
        </script>
    </dom-module>
</body>
</html>