Introduced a linter and Travis.

.travis.yml

+sudo: required
+dist: trusty
+addons:
+  firefox: latest
+  apt:
+    sources:
+    - google-chrome
+    packages:
+    - google-chrome-stable
+language: node_js
+node_js:
+- '8'
+cache:
+- node_modules
+branches:
+  except:
+  - stable
+before_install:
+- export DISPLAY=:99.0
+- sh -e /etc/init.d/xvfb start
+- npm config set package-lock false
+- npm i -g npm@^5.7.1
+install:
+- npm install @ckeditor/ckeditor5-dev-tests
+- ckeditor5-dev-tests-install-dependencies
+script:
+- ckeditor5-dev-tests-travis
+env:
+  global:
+  - secure: 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
+  - secure: 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

package.json

@@ -48,6 +48,10 @@
     "@ckeditor/ckeditor5-table": "^11.0.1",
     "@ckeditor/ckeditor5-theme-lark": "^12.0.0",
     "@ckeditor/ckeditor5-upload": "^10.0.4",
+    "eslint": "^5.5.0",
+    "eslint-config-ckeditor5": "^1.0.7",
+    "husky": "^0.14.3",
+    "lint-staged": "^7.0.0",
     "postcss-loader": "^3.0.0",
     "raw-loader": "^0.5.1",
     "style-loader": "^0.23.0",
@@ -69,6 +73,16 @@
   },
   "scripts": {
     "build": "webpack --mode production",
+    "lint": "eslint --quiet '**/*.js'",
+    "precommit": "lint-staged",
     "preversion": "npm run build; if [ -n \"$(git status src/ckeditor.js build/ --porcelain)\" ]; then git add -u src/ckeditor.js build/ && git commit -m 'Internal: Build.'; fi"
-  }
+  },
+  "lint-staged": {
+    "**/*.js": [
+      "eslint --quiet"
+    ]
+  },
+  "eslintIgnore": [
+    "build/**"
+  ]
 }