Add comment to async* support code

runtime/lib/core_patch.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import "dart:math";
 import "dart:typed_data";
 
 // Equivalent of calling FATAL from C++ code.
 _fatal(msg) native "DartCore_fatal";
 
@@ skipping 43 matching lines @@
       return false;
     }
     while(true) {
       if (yieldEachIterator != null) {
         if (yieldEachIterator.moveNext()) {
           return true;
         }
         yieldEachIterator = null;
       }
       isYieldEach = false;
+      // moveNextFn() will update the values of isYieldEach and _current.
       if (!moveNextFn(this)) {
         moveNextFn = null;
         _current = null;
         return false;
       }
       if (isYieldEach) {
         // Spec mandates: it is a dynamic error if the class of [the object
         // returned by yield*] does not implement Iterable.
         yieldEachIterator = (_current as Iterable).iterator;
         _current = null;
         continue;
       }
       return true;
     }
   }
 }
 
 patch class StackTrace {
   /* patch */ static StackTrace get current native "StackTrace_current";
 }