Polymer/Closure: Step #2, Compile

gulpfile.js

const closure = require('google-closure-compiler-js').gulp();
const fs = require('fs');

gulp.task('compile', ['merge'], function() {
  // nb. This is a bit awkward, but we need to load the Polymer externs so that
  // Closure can compile Polymer code correctly.
  const externsPath =
      'node_modules/google-closure-compiler-js/contrib/externs/polymer-1.0.js';
  const externsSrc = fs.readFileSync(externsPath, 'utf-8');

  return gulp.src('dest/elements.js')
    .pipe(closure({
      polymerPass: true,  // !! IMPORTANT !!
      externs: [
        {'src': externsSrc, 'path': 'polymer-1.0.js'}
      ],
      compilationLevel: 'SIMPLE',
      warningLevel: 'VERBOSE',
      jsOutputFile: 'elements.min.js',
      createSourceMap: true,
    }))
    .pipe(gulp.dest('dest'));
});

Note that this requires Step #1 to work correctly.