Fix issue 1950:

vm/flags.h

Index: vm/flags.h
===================================================================
--- vm/flags.h	(revision 5257)
+++ vm/flags.h	(working copy)
@@ -19,7 +19,10 @@
                                             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)                  \
@@ -31,6 +34,8 @@
 
 namespace dart {
 
+typedef void (*FlagHandler)(bool value);
+
 // Forward declaration.
 class Flag;
 
@@ -51,6 +56,10 @@
                                     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);