js_checker.py: Restore smoke tests for linting violations.

tools/web_dev_style/js_checker.py

Index: tools/web_dev_style/js_checker.py
diff --git a/tools/web_dev_style/js_checker.py b/tools/web_dev_style/js_checker.py
index 7d48c3f853bacc7a347a74a378355139f50f4133..79e0895971cea937e7adae075dfa05ff334f8c73 100644
--- a/tools/web_dev_style/js_checker.py
+++ b/tools/web_dev_style/js_checker.py
@@ -62,7 +62,7 @@ class JSChecker(object):
     return self.RegexCheck(i, line, r"(?<!this)(\.\$)[\[\.]",
         "Please only use this.$.localId, not element.$.localId")
 
-  def RunEsLintChecks(self, affected_js_files):
+  def RunEsLintChecks(self, affected_js_files, format='stylish'):
     """Runs lint checks using ESLint. The ESLint rules being applied are defined
        in the .eslintrc.js configuration file.
     """
@@ -89,6 +89,7 @@ class JSChecker(object):
     output = node.RunNode([
         node_modules.PathToEsLint(),
         '--color',
+        '--format', format,
         '--ignore-pattern \'!.eslintrc.js\'',
         ' '.join(affected_js_files_paths)])