[es6] implement default parameters via desugaring

test/mjsunit/harmony/optional-arguments-debug.js

+// 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.
+
+
+// Flags: --expose-debug-as debug
+
+// Get the Debug object exposed from the debug context global object.
+Debug = debug.Debug
+
+listenerComplete = false;
+breakPointCount = 0;
+
+function listener(event, exec_state, event_data, data) {
+  if (event == Debug.DebugEvent.Break) {
+    breakPointCount++;
+    if (breakPointCount == 1) {
+      // Break point in initializer for parameter `a`, invoked by
+      // initializer for parameter `b`
+      assertEquals('default', exec_state.frame(1).evaluate('mode').value());
+
+      // initializer for `b` can't refer to `b`
+      assertThrows(function() {
+        return exec_state.frame(1).evaluate('b').value();
+      }, ReferenceError);
+
+      assertThrows(function() {
+        return exec_state.frame(1).evaluate('c');
+      }, ReferenceError);
+    } else if (breakPointCount == 2) {
+      // Break point in IIFE initializer for parameter `c`
+      assertEquals('modeFn', exec_state.frame(1).evaluate('a.name').value());
+      assertEquals('default', exec_state.frame(1).evaluate('b').value());
+      assertThrows(function() {
+        return exec_state.frame(1).evaluate('c');
+      }, ReferenceError);
+    } else if (breakPointCount == 3) {
+      // Break point in function body --- `c` parameter is shadowed
+      assertEquals('modeFn', exec_state.frame(0).evaluate('a.name').value());
+      assertEquals('default', exec_state.frame(0).evaluate('b').value());
+      // TODO(caitp): fix scoping so that parameter `c` can be shadowed by vars
+      //assertEquals(true, exec_state.frame(0).evaluate('c').value());
+    }
+  }
+};
+
+// Add the debug event listener.
+Debug.setListener(listener);
+
+function f(a = function modeFn(mode) {
+                 debugger;
+                 return mode;
+               },
+           b = a("default"),
+           c = (function() {
+             debugger;
+           })()) {
+  // TODO(caitp): fix scoping so that parameter `c` can be shadowed by vars
+  //var c = true;
+  debugger;
+};
+
+f();
+
+// Make sure that the debug event listener vas invoked.
+assertEquals(3, breakPointCount);