Add lint_test.sh and corresponding bazel test

ESLint can now be run from Bazel using...

> bazel test //polygerrit-ui/app:lint_test
...or the maybe more user-friendly...
> bazel test //polygerrit-ui/app:lint_test --test_output errors

This script has several limitations, and is meant mostly as an MVP while
the PG codebase itself still doesn't meet the linter's standards.

- It is not hermetic (it cannot download and run the necessary binaries
  in a sandboxed environment)
- The user cannot specify which files they want to test
- The user cannot specify whether they want to fix any errors
  automatically
- The user cannot pass any other arbitrary flags to ESLint
- CI does not run this test when applying the Verified label

These issues will all be fixed in later changes.

Bug: Issue 6163
Change-Id: I14b8d165600d6c3f9394c2bb6ec520e29e40b308

polygerrit-ui/app/BUILD

@@ -157,3 +157,18 @@ sh_test(
         "manual",
     ],
 )
+
+sh_test(
+    name = "lint_test",
+    size = "large",
+    srcs = ["lint_test.sh"],
+    data = [
+        ":pg_code",
+        ".eslintrc.json",
+    ],
+    # Should not run sandboxed.
+    tags = [
+        "local",
+        "manual",
+    ],
+)

polygerrit-ui/app/lint_test.sh

@@ -0,0 +1,22 @@
+#!/bin/sh
+
+set -ex
+
+eslint_bin=$(which npm)
+if [[ -z "$eslint_bin" ]]; then
+    echo "NPM must be on the path."
+    exit 1
+fi
+
+eslint_bin=$(which eslint)
+eslint_config=$(npm list -g | grep -c eslint-config-google)
+eslint_plugin=$(npm list -g | grep -c eslint-plugin-html)
+if [[ -z "$eslint_bin" ]] || [[ eslint_config -eq "0" ]] || [[ eslint_plugin -eq "0" ]]; then
+    echo "You must install ESLint and its dependencies from NPM."
+    echo "> npm install -g eslint eslint-config-google eslint-plugin-html"
+    echo "For more information, view the README:"
+    echo "https://gerrit.googlesource.com/gerrit/+/master/polygerrit-ui/#Style-guide"
+    exit 1
+fi
+
+${eslint_bin} --ignore-pattern 'bower_components/' --ignore-pattern 'gr-linked-text' --ignore-pattern 'scripts/vendor' --ext .html,.js .