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

runtime/vm/debugger.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 "vm/debugger.h"
 
 #include "include/dart_api.h"
 
 #include "vm/code_generator.h"
 #include "vm/code_patcher.h"
@@ skipping 26 matching lines @@
 DEFINE_FLAG(bool, trace_debugger_stacktrace, false,
             "Trace debugger stacktrace collection");
 DEFINE_FLAG(bool, verbose_debug, false, "Verbose debugger messages");
 DEFINE_FLAG(bool, steal_breakpoints, false,
             "Intercept breakpoints and other pause events before they "
             "are sent to the embedder and use a generic VM breakpoint "
             "handler instead.  This handler dispatches breakpoints to "
             "the VM service.");
 
 DECLARE_FLAG(bool, trace_isolates);
+DECLARE_FLAG(bool, warn_on_pause_with_no_debugger);
 
 
 Debugger::EventHandler* Debugger::event_handler_ = NULL;
 
 
 class RemoteObjectCache : public ZoneAllocated {
  public:
   explicit RemoteObjectCache(intptr_t initial_size);
   intptr_t AddObject(const Object& obj);
   RawObject* GetObj(intptr_t obj_id) const;
@@ skipping 217 matching lines @@
 static bool ServiceNeedsDebuggerEvent(DebuggerEvent::EventType type) {
   switch (type) {
     case DebuggerEvent::kBreakpointResolved:
       // kBreakpointResolved events are handled differently in the vm
       // service, so suppress them here.
       return false;
 
     case DebuggerEvent::kBreakpointReached:
     case DebuggerEvent::kExceptionThrown:
     case DebuggerEvent::kIsolateInterrupted:
-      return Service::debug_stream.enabled();
+      return (Service::debug_stream.enabled() ||
+              FLAG_warn_on_pause_with_no_debugger);
 
     case DebuggerEvent::kIsolateCreated:
     case DebuggerEvent::kIsolateShutdown:
       return Service::isolate_stream.enabled();
 
     default:
       UNREACHABLE();
       return false;
   }
 }
@@ skipping 2967 matching lines @@
 }
 
 
 void Debugger::RegisterCodeBreakpoint(CodeBreakpoint* bpt) {
   ASSERT(bpt->next() == NULL);
   bpt->set_next(code_breakpoints_);
   code_breakpoints_ = bpt;
 }
 
 }  // namespace dart