tools: lint rule for assert.fail()

`assert.fail()` is often mistakenly used with a single argument even in
Node.js core. (See fixes to previous instances in
b7f4b1ba4c,
28e9a022df. and
676e61872f54dd546e324599c7871c20b798386a.)

This commit adds a linting rule to identify instances of this issue.

PR-URL: https://github.com/nodejs/node/pull/6261
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

.eslintrc

@@ -85,6 +85,7 @@ rules:
   prefer-const: 2
 
   # Custom rules in tools/eslint-rules
+  assert-fail-single-argument: 2
   new-with-error: [2, "Error", "RangeError", "TypeError", "SyntaxError", "ReferenceError"]
   align-multiline-assignment: 2
 

tools/eslint-rules/assert-fail-single-argument.js

@@ -0,0 +1,30 @@
+/**
+ * @fileoverview Prohibit use of a single argument only in `assert.fail()`. It
+ * is almost always an error.
+ * @author Rich Trott
+ */
+'use strict';
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+const msg = 'assert.fail() message should be third argument';
+
+function isAssert(node) {
+  return node.callee.object && node.callee.object.name === 'assert';
+}
+
+function isFail(node) {
+  return node.callee.property && node.callee.property.name === 'fail';
+}
+
+module.exports = function(context) {
+  return {
+    'CallExpression': function(node) {
+      if (isAssert(node) && isFail(node) && node.arguments.length === 1) {
+        context.report(node, msg);
+      }
+    }
+  };
+};