Added option --shell to D8 and updated the usage text.

src/d8.cc

 // Copyright 2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 370 matching lines @@
 int Shell::Main(int argc, char* argv[]) {
   i::FlagList::SetFlagsFromCommandLine(&argc, argv, true);
   if (i::FLAG_help) {
     return 1;
   }
   Initialize();
   bool run_shell = (argc == 1);
   Context::Scope context_scope(evaluation_context_);
   for (int i = 1; i < argc; i++) {
     char* str = argv[i];
-    if (strcmp(str, "-f") == 0) {
+    if (strcmp(str, "--shell") == 0) {
+      run_shell = true;
+    } else if (strcmp(str, "-f") == 0) {
       // Ignore any -f flags for compatibility with other stand-alone
       // JavaScript engines.
       continue;
     } else if (strncmp(str, "--", 2) == 0) {
       printf("Warning: unknown flag %s.\nTry --help for options\n", str);
     } else if (strcmp(str, "-e") == 0 && i + 1 < argc) {
       // Execute argument given to -e option directly.
       v8::HandleScope handle_scope;
       v8::Handle<v8::String> file_name = v8::String::New("unnamed");
       v8::Handle<v8::String> source = v8::String::New(argv[i + 1]);
@@ skipping 19 matching lines @@
   return 0;
 }
 
 
 }  // namespace v8
 
 
 int main(int argc, char* argv[]) {
   return v8::Shell::Main(argc, argv);
 }