Support stepping into generator function.

src/generator.js

 // Copyright 2013 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.
 
 "use strict";
 
 // This file relies on the fact that the following declarations have been made
 // in runtime.js:
 // var $Function = global.Function;
 
 // ----------------------------------------------------------------------------
 
 
 // Generator functions and objects are specified by ES6, sections 15.19.3 and
 // 15.19.4.
 
 function GeneratorObjectNext(value) {
   if (!IS_GENERATOR(this)) {
     throw MakeTypeError('incompatible_method_receiver',
                         ['[Generator].prototype.next', this]);
   }
 
+  if (DEBUG_IS_ACTIVE) %DebugPrepareStepInIfStepping(this);
   return %_GeneratorNext(this, value);
 }
 
 function GeneratorObjectThrow(exn) {
   if (!IS_GENERATOR(this)) {
     throw MakeTypeError('incompatible_method_receiver',
                         ['[Generator].prototype.throw', this]);
   }
 
   return %_GeneratorThrow(this, exn);
@@ skipping 44 matching lines @@
       GeneratorFunctionPrototype, DONT_ENUM | DONT_DELETE | READ_ONLY);
   %InternalSetPrototype(GeneratorFunctionPrototype, $Function.prototype);
   %SetCode(GeneratorFunctionPrototype, GeneratorFunctionPrototypeConstructor);
   %AddNamedProperty(GeneratorFunctionPrototype, "constructor",
       GeneratorFunction, DONT_ENUM | DONT_DELETE | READ_ONLY);
   %InternalSetPrototype(GeneratorFunction, $Function);
   %SetCode(GeneratorFunction, GeneratorFunctionConstructor);
 }
 
 SetUpGenerators();