Add --warn-on-pause-with-no-debugger flag. Add this to --observe behavior.

runtime/bin/main.cc

 // 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.
 
 #include <stdlib.h>
 #include <string.h>
 #include <stdio.h>
 
 #include "include/dart_api.h"
 #include "include/dart_tools_api.h"
@@ skipping 407 matching lines @@
                         &vm_service_server_port,
                         &vm_service_server_ip,
                         DEFAULT_VM_SERVICE_SERVER_PORT,
                         DEFAULT_VM_SERVICE_SERVER_IP)) {
     Log::PrintErr("unrecognized --observe option syntax. "
                   "Use --observe[:<port number>[/<IPv4 address>]]\n");
     return false;
   }
 
   vm_options->AddArgument("--pause-isolates-on-exit");
+  vm_options->AddArgument("--pause-isolates-on-unhandled-exceptions");
+  vm_options->AddArgument("--warn-on-pause-with-no-debugger");
   return true;
 }
 
 
 static bool ProcessTraceLoadingOption(const char* arg,
                                       CommandLineOptions* vm_options) {
   if (*arg != '\0') {
     return false;
   }
   has_trace_loading = true;
@@ skipping 415 matching lines @@
 "--checked or -c\n"
 "  Insert runtime type checks and enable assertions (checked mode).\n"
 "--help or -h\n"
 "  Display this message (add -v or --verbose for information about\n"
 "  all VM options).\n"
 "--package-root=<path> or -p<path>\n"
 "  Where to find packages, that is, \"package:...\" imports.\n"
 "--packages=<path>\n"
 "  Where to find a package spec file.\n"
 "--observe[=<port>[/<bind-address>]]\n"
-"  Enable the VM service and cause isolates to pause on exit (default port is\n"
-"  8181, default bind address is 127.0.0.1). With the default options,\n"
-"  Observatory will be available locally at http://127.0.0.1:8181/\n"
+"  The observe flag is used to run a program with a default set of options\n"
+"  for debugging under Observatory. With the default options, Observatory\n"
+"  will be available at http://127.0.0.1:8181/ (default port is 8181,\n"
+"  default bind address is 127.0.0.1).  Isolates will pause at exit and\n"
+"  when they throw unhandled exceptions.\n"
 "--version\n"
 "  Print the VM version.\n");
   } else {
     Log::PrintErr(
 "Supported options:\n"
 "--checked or -c\n"
 "  Insert runtime type checks and enable assertions (checked mode).\n"
 "--help or -h\n"
 "  Display this message (add -v or --verbose for information about\n"
 "  all VM options).\n"
 "--package-root=<path> or -p<path>\n"
 "  Where to find packages, that is, \"package:...\" imports.\n"
 "--packages=<path>\n"
 "  Where to find a package spec file.\n"
 "--observe[=<port>[/<bind-address>]]\n"
-"  Enable the VM service and cause isolates to pause on exit (default port is\n"
-"  8181, default bind address is 127.0.0.1). With the default options,\n"
-"  Observatory will be available locally at http://127.0.0.1:8181/\n"
+"  The observe flag is used to run a program with a default set of options\n"
+"  for debugging under Observatory. With the default options, Observatory\n"
+"  will be available at http://127.0.0.1:8181/ (default port is 8181,\n"
+"  default bind address is 127.0.0.1).  Isolates will pause at exit and\n"
+"  when they throw unhandled exceptions.\n"
 "--version\n"
 "  Print the VM version.\n"
 "\n"
 "--snapshot=<file_name>\n"
 "  loads Dart script and generates a snapshot in the specified file\n"
 "\n"
 "--trace-loading\n"
 "  enables tracing of library and script loading\n"
 "\n"
-"--enable-vm-service[:<port number>]\n"
+"--enable-vm-service[:<port>[/<bind-address>]]\n"
 "  enables the VM service and listens on specified port for connections\n"
-"  (default port number is 8181)\n"
+"  (default port number is 8181, default bind address is 127.0.0.1).\n"
 "\n"
 "The following options are only used for VM development and may\n"
 "be changed in any future version:\n");
     const char* print_flags = "--print_flags";
     Dart_SetVMFlags(1, &print_flags);
   }
 }
 
 
 char* BuildIsolateName(const char* script_name,
@@ skipping 625 matching lines @@
   Platform::Exit(Process::GlobalExitCode());
 }
 
 }  // namespace bin
 }  // namespace dart
 
 int main(int argc, char** argv) {
   dart::bin::main(argc, argv);
   UNREACHABLE();
 }