Make syntax for boolean flags more discoverable.

src/flags.cc

 // Copyright 2006-2008 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.
 
 #include "src/flags.h"
 
 #include <cctype>
 #include <cstdlib>
 #include <sstream>
 
@@ skipping 411 matching lines @@
       }
 
       // handle errors
       bool is_bool_type = flag->type() == Flag::TYPE_BOOL ||
           flag->type() == Flag::TYPE_MAYBE_BOOL;
       if ((is_bool_type && value != NULL) || (!is_bool_type && is_bool) ||
           *endp != '\0') {
         PrintF(stderr, "Error: illegal value for flag %s of type %s\n"
                "Try --help for options\n",
                arg, Type2String(flag->type()));
+        if (is_bool_type) {
+          PrintF(stderr,
+                 "To set or unset a boolean flag, use --flag or --no-flag.\n");
+        }
         return_code = j;
         break;
       }
 
       // remove the flag & value from the command
       if (remove_flags) {
         while (j < i) {
           argv[j++] = NULL;
         }
       }
@@ skipping 130 matching lines @@
 #define FLAG_MODE_DEFINE_IMPLICATIONS
 #include "src/flag-definitions.h"  // NOLINT(build/include)
 #undef FLAG_MODE_DEFINE_IMPLICATIONS
   ComputeFlagListHash();
 }
 
 
 uint32_t FlagList::Hash() { return flag_hash; }
 }  // namespace internal
 }  // namespace v8