maximumLineLength: improve functionSignature option

Add spec for an arrow function expression, and fix it.

Treat `ArrowFunctionExpression` same way as `FunctionExpression`

Fixes #2032
Closes gh-2040

Author: himdel

lib/rules/maximum-line-length.js

@@ -37,6 +37,34 @@
  * ```js
  * var aLineOf41Chars = 1234567890123456789;
  * ```
+ *
+ * #### Example for allExcept functionSignature
+ *
+ * ```js
+ * "maximumLineLength": { "value": 40, "allExcept": [ "functionSignature" ] }
+ * ```
+ *
+ * ##### Valid
+ *
+ * ```js
+ * var f = function(with, many, _many_, arguments) { .... };
+ * let f = x => x * x * x * x * x * x * x * x;
+ * (function(foo, bar, baz, quux, cuttlefish) {
+ *     function namesNaamesNaaamesNaaaames() {
+ *         ...
+ *     }
+ * })();
+ * const longNameIgnoredAsWell = (a, b) => a * b;
+ * class X { myLongMethodName(withPossiblyManyArgs) { ... } };
+ * ```
+ *
+ * ##### Invalid
+ *
+ * ```js
+ * function x() { // valid
+ *     return "function_bodies_are_not_protected";
+ * }
+ * ```
  */
 
 var assert = require('assert');
@@ -134,11 +162,11 @@ module.exports.prototype = {
 
             file.iterateNodesByType('MethodDefinition', function(node) {
                 removeLoc(node.key);
-                // node.value is a FunctionExpression, params are handled there
             });
 
-            file.iterateNodesByType('FunctionExpression', function(node) {
-                // need to remove the first line, because we can't be sure there's any id or params
+            file.iterateNodesByType(['ArrowFunctionExpression', 'FunctionExpression'], function(node) {
+
+                // Need to remove the first line, because we can't be sure there's any id or params
                 lines[node.loc.start.line - 1] = '';
                 if (node.id) {
                     removeLoc(node.id);

test/specs/rules/maximum-line-length.js

@@ -224,6 +224,13 @@ describe('rules/maximum-line-length', function() {
             expect(checker.checkString(code)).to.have.no.errors();
         });
 
+        it('should not report arrow functions', function() {
+            var code = '(aVeryVeryLongLongParameter => 42);\n' +
+                    '(() => "parameterless arrow function");\n' +
+                    '((foo, bar, baz, $rootScope) => "quux");';
+            expect(checker.checkString(code)).to.have.no.errors();
+        });
+
         it('should not report functions within IIFE blocks', function() {
             var code = '(function() {\n' +
                     '   function myCoolFunction(argument) { }\n' +