Refactor isolate interrupts to use OOB messages instead of interrupt bits.

runtime/observatory/lib/src/elements/debugger.dart

Index: runtime/observatory/lib/src/elements/debugger.dart
diff --git a/runtime/observatory/lib/src/elements/debugger.dart b/runtime/observatory/lib/src/elements/debugger.dart
index 66c960987911fdb5750ba2ea4854b2000867c166..a90dbd1b386790908c2f6fe6902d1cfbdf442e50 100644
--- a/runtime/observatory/lib/src/elements/debugger.dart
+++ b/runtime/observatory/lib/src/elements/debugger.dart
@@ -1384,6 +1384,9 @@ class ObservatoryDebugger extends Debugger {
           console.print('Paused at ${script.name}:${line}:${col}');
         }
       });
+    } else {
+      console.print("Paused in message loop (type 'continue' or [F7] "
+                    "to resume processing messages)");
     }
   }