Fix issue 1950:

vm/flags.h

 // Copyright (c) 2012, 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.
 
 #ifndef VM_FLAGS_H_
 #define VM_FLAGS_H_
 
 #include "platform/assert.h"
 #include "vm/globals.h"
 
 typedef const char* charp;
 
 #define DECLARE_FLAG(type, name)                                               \
   extern type FLAG_##name
 
 #define DEFINE_FLAG(type, name, default_value, comment)                        \
   type FLAG_##name = Flags::Register_##type(&FLAG_##name,                      \
                                             #name,                             \
                                             default_value,                     \
                                             comment)
 
+#define DEFINE_FLAG_HANDLER(handler, name, comment)                            \
+  bool DUMMY_##name = Flags::Register_func(handler, #name, comment)
+
 
 #if defined(DEBUG)
 #define DECLARE_DEBUG_FLAG(type, name) DECLARE_FLAG(type, name)
 #define DEFINE_DEBUG_FLAG(type, name, default_value, comment)                  \
   DEFINE_FLAG(type, name, default_value, comment)
 #else
 #define DECLARE_DEBUG_FLAG(type, name)
 #define DEFINE_DEBUG_FLAG(type, name, default_value, comment)
 #endif
 
 namespace dart {
 
+typedef void (*FlagHandler)(bool value);
+
 // Forward declaration.
 class Flag;
 
 class Flags {
  public:
   static bool Register_bool(bool* addr,
                             const char* name,
                             bool default_value,
                             const char* comment);
 
   static int Register_int(int* addr,
                           const char* name,
                           int default_value,
                           const char* comment);
 
   static const char* Register_charp(charp* addr,
                                     const char* name,
                                     const char* default_value,
                                     const char* comment);
 
+  static bool Register_func(FlagHandler handler,
+                            const char* name,
+                            const char* comment);
+
   static bool ProcessCommandLineFlags(int argc, const char** argv);
 
   static Flag* Lookup(const char* name);
 
   static bool Initialized() { return initialized_; }
 
  private:
   static Flag* flags_;
 
   static bool initialized_;
 
   static void Parse(const char* option);
 
   // Testing needs direct access to private methods.
   friend void Dart_TestParseFlags();
 
   DISALLOW_ALLOCATION();
   DISALLOW_IMPLICIT_CONSTRUCTORS(Flags);
 };
 
 }  // namespace dart
 
 #endif  // VM_FLAGS_H_