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

runtime/vm/service_event.h

 // Copyright (c) 2015, 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.
 
 #ifndef VM_SERVICE_EVENT_H_
 #define VM_SERVICE_EVENT_H_
 
 #include "vm/debugger.h"
 
 class DebuggerEvent;
@@ skipping 52 matching lines @@
   };
 
   ServiceEvent(Isolate* isolate, EventKind event_kind);
 
   explicit ServiceEvent(const DebuggerEvent* debugger_event);
 
   Isolate* isolate() const { return isolate_; }
 
   EventKind kind() const { return kind_; }
 
+  bool IsPause() const {
+    switch (kind())  {
+      case kPauseStart:
+      case kPauseExit:
+      case kPauseBreakpoint:
+      case kPauseInterrupted:
+      case kPauseException:
+        return true;
+      default:
+        return false;
+    }
+  }
+
   const char* embedder_kind() const { return embedder_kind_; }
 
   const char* KindAsCString() const;
 
   void set_embedder_kind(const char* embedder_kind) {
     embedder_kind_ = embedder_kind;
   }
 
   const char* stream_id() const;
 
@@ skipping 114 matching lines @@
   const uint8_t* bytes_;
   intptr_t bytes_length_;
   LogRecord log_record_;
   ExtensionEvent extension_event_;
   int64_t timestamp_;
 };
 
 }  // namespace dart
 
 #endif  // VM_SERVICE_EVENT_H_