Fix for issue 6768

runtime/bin/gen_snapshot.cc

Index: runtime/bin/gen_snapshot.cc
===================================================================
--- runtime/bin/gen_snapshot.cc	(revision 15751)
+++ runtime/bin/gen_snapshot.cc	(working copy)
@@ -135,6 +135,11 @@
     *script_name = NULL;
   }
 
+  if (snapshot_filename == NULL) {
+    Log::PrintErr("No snapshot output file specified.\n\n");
+    return -1;
+  }
+
   return 0;
 }
 
@@ -358,7 +363,29 @@
 
 
 static void PrintUsage() {
-  Log::PrintErr("dart [<vm-flags>] [<dart-script-file>]\n");
+  Log::PrintErr(
+"Usage:\n"
+"\n"
+"  gen_snapshot [<vm-flags>] [<options>] \\\n"
+"               {--script_snapshot=<out-file> | --snapshot=<out-file>} \\\n"
+"               [<dart-script-file>]\n"
+"\n"
+"  Writes a snapshot of <dart-script-file> to <out-file>. If no\n"
+"  <dart-script-file> is passed, a generic snapshot of all the corelibs is\n"
+"  created. One of the following is required:\n"
+"\n"
+"    --script_snapshot=<file>   Generates a script snapshot.\n"
+"    --snapshot=<file>          Generates a complete snapshot. Uses the url\n"
+"                               mapping specified on the command line to load\n"
+"                               the libraries.\n"
+"Supported options:\n"
+"\n"
+"--package_root=<path>\n"
+"  Where to find packages, that is, \"package:...\" imports.\n"
+"\n"
+"--url_mapping=<mapping>\n"
+"  Uses the URL mapping(s) specified on the command line to load the\n"
+"  libraries. For use only with --snapshot=.\n");
 }
 
 
@@ -447,11 +474,6 @@
     return 255;
   }
 
-  if (snapshot_filename == NULL) {
-    Log::PrintErr("No snapshot output file specified\n");
-    return 255;
-  }
-
   DartUtils::SetOriginalWorkingDirectory();
 
   Dart_SetVMFlags(vm_options.count(), vm_options.arguments());