[es6] implement default parameters via desugaring

test/mjsunit/harmony/default-parameters-lazy.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: --harmony-default-parameters --min-preparse-length=0
+
+var i = 0;
+function f(handler = function(b) { return b + "#" + (++i); }, b = "red") {
+  return handler(b);
+}
+
+assertEquals([
+  "blue#1",
+  "red#2",
+  "red",
+  "yellow#3"
+], [
+  f(undefined, "blue"),
+  f(),
+  f(function(b) { return b; }),
+  f(undefined, "yellow")
+]);