[es6] implement optional parameters via desugaring (with scoping)

test/mjsunit/harmony/optional-arguments.js

Index: test/mjsunit/harmony/optional-arguments.js
diff --git a/test/mjsunit/harmony/optional-arguments.js b/test/mjsunit/harmony/optional-arguments.js
new file mode 100644
index 0000000000000000000000000000000000000000..d0fe0bbe239c5a3472d51a38004478d7c55f68b3
--- /dev/null
+++ b/test/mjsunit/harmony/optional-arguments.js
@@ -0,0 +1,67 @@
+// Copyright 2015 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+
+function return_specified() { return "specified"; }
+
+var method_returns_specified = {
+  method() { return "specified"; }
+};
+
+
+(function testOptionalFunctions() {
+  function optional_function(handler = function() { }) {
+    assertEquals("function", typeof handler);
+
+    // TODO(caitp): infer function name correctly
+    // assertEquals("handler", handler.name);
+
+    return handler();
+  }
+  assertEquals(undefined, optional_function());
+  assertEquals(undefined, optional_function(undefined));
+  assertEquals("specified", optional_function(return_specified));
+})();
+
+
+(function testOptionalObjects() {
+  function optional_object(object = { method() { return "method"; } }) {
+    assertEquals("object", typeof object);
+
+    assertEquals("function", typeof object.method);
+    return object.method();
+  }
+
+  assertEquals("method", optional_object());
+  assertEquals("method", optional_object(undefined));
+  assertEquals("specified", optional_object(method_returns_specified));
+})();
+
+
+// TDZ
+
+(function testReferencesUninitializedParameter(x = 1) {
+  assertThrows(function(a = b, b) {}, ReferenceError);
+})();
+
+
+(function testReferencesInitializedParameter(x = 1) {
+  assertEquals(1, (function(a = 1, b = a) { return b; })());
+})();
+
+
+// Scoping
+assertThrows(function referencesVariableBodyDeclaration(a = body_var) {
+  var body_var = true;
+  return a;
+}, ReferenceError);
+
+
+// TODO(caitp): default function length does not include any parameters
+// following the first optional parameter
+// assertEquals(0, (function(a = 1) {}).length);
+// assertEquals(1, (function(a, b = 1) {}).length);
+// assertEquals(2, (function(a, b, c = 1) {}).length);
+// assertEquals(3, (function(a, b, c, d = 1) {}).length);
+// assertEquals(1, (function(a, b = 1, c, d = 1) {}).length);