Fix static analysis warnings in Observatory

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 fdac4df9f6c4f5dc4a1a8f3e4a2c9420a3eef43f..66c960987911fdb5750ba2ea4854b2000867c166 100644
--- a/runtime/observatory/lib/src/elements/debugger.dart
+++ b/runtime/observatory/lib/src/elements/debugger.dart
@@ -1655,16 +1655,16 @@ class ObservatoryDebugger extends Debugger {
       var event = isolate.pauseEvent;
       if (event.kind == ServiceEvent.kPauseStart) {
         console.print("Type 'continue' [F7] or 'step' [F10] to start the isolate");
-        return;
+        return null;
       }
       if (event.kind == ServiceEvent.kPauseExit) {
         console.print("Type 'continue' [F7] to exit the isolate");
-        return;
+        return null;
       }
       return isolate.stepOver();
     } else {
       console.print('The program is already running');
-      return;
+      return null;
     }
   }