Add test coverage threshold

.gitignore

@@ -1,3 +1,4 @@
 bower_components
 node_modules
 package-lock.json
+coverage

.travis.yml

@@ -39,3 +39,6 @@ script:
         wct --env saucelabs-cron;
       fi;
     fi
+
+after_success:
+  - "cat ${TRAVIS_BUILD_DIR}/coverage/lcov.info | coveralls"

README.md

@@ -1,6 +1,7 @@
 [![npm version](https://badge.fury.io/js/%40vaadin%2Fvaadin-text-field.svg)](https://badge.fury.io/js/%40vaadin%2Fvaadin-text-field)
 [![Published on webcomponents.org](https://img.shields.io/badge/webcomponents.org-published-blue.svg)](https://www.webcomponents.org/element/vaadin/vaadin-text-field)
 [![Build Status](https://travis-ci.org/vaadin/vaadin-text-field.svg?branch=master)](https://travis-ci.org/vaadin/vaadin-text-field)
+[![Coverage Status](https://coveralls.io/repos/github/vaadin/vaadin-text-field/badge.svg?branch=master)](https://coveralls.io/github/vaadin/vaadin-text-field?branch=master)
 [![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/vaadin/web-components?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)
 
 [![Published on Vaadin  Directory](https://img.shields.io/badge/Vaadin%20Directory-published-00b4f0.svg)](https://vaadin.com/directory/component/vaadinvaadin-text-field)

package.json

@@ -23,6 +23,7 @@
   },
   "devDependencies": {
     "bower": "latest",
+    "coveralls": "^3.0.1",
     "eslint": "^4.0.0",
     "eslint-config-vaadin": "latest",
     "eslint-plugin-html": "^4.0.0",
@@ -39,6 +40,7 @@
     "polymer-cli": "^1.6.0",
     "stylelint": "^9.0.0",
     "stylelint-config-vaadin": "latest",
+    "wct-istanbul": "^0.14.3",
     "web-component-tester": "^6.1.5",
     "yargs": "^8.0.0"
   }

wct.conf.js

@@ -1,6 +1,22 @@
 var argv = require('yargs').argv;
 
 module.exports = {
+  plugins: {
+    'istanbul': {
+      dir: './coverage',
+      reporters: ['text-summary', 'lcov'],
+      include: [
+        '**/vaadin-text-field/src/*.html'
+      ],
+      exclude: [],
+      thresholds: {
+        global: {
+          statements: 87
+        }
+      }
+    }
+  },
+
   registerHooks: function(context) {
     var saucelabsPlatforms = [
       'macOS 10.12/iphone@10.3',