Fix static analysis warnings in Observatory

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

 // Copyright (c) 2014, 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.
 
 library debugger_page_element;
 
 import 'dart:async';
 import 'dart:html';
 import 'observatory_element.dart';
 import 'package:observatory/app.dart';
@@ skipping 1637 matching lines @@
     } else {
       console.print('The program is already running');
     }
   }
 
   Future syncNext() async {
     if (isolatePaused()) {
       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;
     }
   }
 
   Future step() {
     if (isolatePaused()) {
       var event = isolate.pauseEvent;
       if (event.kind == ServiceEvent.kPauseExit) {
         console.print("Type 'continue' [F7] to exit the isolate");
         return new Future.value(null);
       }
@@ skipping 678 matching lines @@
         }
       });
   }
 
   void focus() {
     $['textBox'].focus();
   }
 
   DebuggerInputElement.created() : super.created();
 }