support trailing commas in function parameter lists and calls (#2156)

fixes #2155

test/input/invalid/sequence.js

@@ -0,0 +1 @@
+(a, ...b);

test/mocha/cli.js

@@ -596,6 +596,21 @@ describe("bin/uglifyjs", function () {
             done();
         });
     });
+    it("Should throw syntax error (spread in sequence)", function(done) {
+        var command = uglifyjscmd + ' test/input/invalid/sequence.js';
+
+        exec(command, function (err, stdout, stderr) {
+            assert.ok(err);
+            assert.strictEqual(stdout, "");
+            assert.strictEqual(stderr.split(/\n/).slice(0, 4).join("\n"), [
+                "Parse error at test/input/invalid/sequence.js:1,4",
+                "(a, ...b);",
+                "    ^",
+                "ERROR: Unexpected token: expand (...)"
+            ].join("\n"));
+            done();
+        });
+    });
     it("Should handle literal string as source map input", function(done) {
         var command = [
             uglifyjscmd,

test/mocha/function.js

@@ -191,15 +191,51 @@ describe("Function", function() {
         ];
         var test = function(code) {
             return function() {
-                uglify.parse(code);
+                uglify.parse(code, { ecma: 5 });
             }
         }
         var error = function(e) {
-            return e instanceof uglify.JS_Parse_Error &&
-                e.message === "Invalid function parameter";
+            return e instanceof uglify.JS_Parse_Error;
         }
         for (var i = 0; i < tests.length; i++) {
-            assert.throws(test(tests[i]), error);
+            assert.throws(test(tests[i]), error, tests[i]);
+        }
+    });
+    it("Should accept trailing commas only for ES8", function() {
+        [
+            "new Foo(a, );",
+            "async(...[1, 2], );",
+            "console.log(...[1, 2], );",
+            "!function(a, b, ){ console.log(a + b); }(3, 4, );",
+        ].forEach(function(code) {
+            uglify.parse(code, { ecma: 8 });
+            assert.throws(function() {
+                uglify.parse(code, { ecma: 6 });
+            }, function(e) {
+                return e instanceof uglify.JS_Parse_Error;
+            }, code);
+        });
+    });
+    it("Should not accept invalid trailing commas", function() {
+        var tests = [
+            "f(, );",
+            "(, ) => {};",
+            "(...p, ) => {};",
+            "function f(, ) {}",
+            "function f(...p, ) {}",
+            "function foo(a, b, , ) {}",
+            'console.log("hello", , );',
+        ];
+        var test = function(code) {
+            return function() {
+                uglify.parse(code, { ecma: 8 });
+            }
+        }
+        var error = function(e) {
+            return e instanceof uglify.JS_Parse_Error;
+        }
+        for (var i = 0; i < tests.length; i++) {
+            assert.throws(test(tests[i]), error, tests[i]);
         }
     });
     it("Should not accept an initializer when parameter is a rest parameter", function() {