Add scope test for arguments

test/mocha/arguments.js

@@ -0,0 +1,19 @@
+var UglifyJS = require('../../');
+var assert = require("assert");
+
+describe("arguments", function() {
+    it("Should known that arguments in functions are local scoped", function() {
+        var ast = UglifyJS.parse("var arguments; var f = function() {arguments.length}");
+        ast.figure_out_scope();
+
+        // Select symbol in function
+        var symbol = ast.body[1].definitions[0].value.find_variable("arguments");
+
+        assert.strictEqual(symbol.global, false);
+        assert.strictEqual(symbol.scope, ast. // From ast
+            body[1]. // Select 2nd statement (equals to `var f ...`)
+            definitions[0]. // First definition of selected statement
+            value // Select function as scope
+        );
+    });
+});