Include a static getter for the element version

.gitignore

@@ -1,2 +1,3 @@
 bower_components
 node_modules
+package-lock.json

.travis.yml

@@ -16,7 +16,7 @@ install:
   - bower install
 
 before_script:
-  - gulp lint
+  - gulp lint version:check
   - polymer lint --rules polymer-2 --input *.html
   - xvfb-run -s '-screen 0 1024x768x24' polymer test
 

gulpfile.js

@@ -4,6 +4,12 @@ var gulp = require('gulp');
 var eslint = require('gulp-eslint');
 var htmlExtract = require('gulp-html-extract');
 var stylelint = require('gulp-stylelint');
+var find = require('gulp-find');
+var replace = require('gulp-replace');
+var expect = require('gulp-expect-file');
+var grepContents = require('gulp-grep-contents');
+var clip = require('gulp-clip-empty-files');
+var git = require('gulp-git');
 
 gulp.task('lint', ['lint:js', 'lint:html', 'lint:css']);
 
@@ -47,3 +53,31 @@ gulp.task('lint:css', function() {
       ]
     }));
 });
+
+gulp.task('version:check', function() {
+  const expectedVersion = new RegExp('^' + require('./package.json').version + '$');
+  return gulp.src(['*.html'])
+    .pipe(htmlExtract({sel: 'script'}))
+    .pipe(find(/static get version.*\n.*/))
+    .pipe(clip()) // Remove non-matching files
+    .pipe(replace(/.*\n.*return '(.*)'.*/, '$1'))
+    .pipe(grepContents(expectedVersion, {invert: true}))
+    .pipe(expect({reportUnexpected: true}, []));
+});
+
+gulp.task('version:update', ['version:check'], function() {
+  // Should be run from 'preversion'
+  // Assumes that the old version is in package.json and the new version in the `npm_package_version` environment variable
+  const oldversion = require('./package.json').version;
+  const newversion = process.env.npm_package_version;
+  if (!oldversion) {
+    throw new 'No old version found in package.json';
+  }
+  if (!newversion) {
+    throw new 'New version must be given as a npm_package_version environment variable.';
+  }
+  return gulp.src(['*.html'])
+    .pipe(replace(oldversion, newversion))
+    .pipe(gulp.dest('.'))
+    .pipe(git.add());
+});