Fix the continue command.

BUG=

Review URL: https://codereview.chromium.org//1271183004 .
diff --git a/runtime/observatory/lib/src/elements/debugger.dart b/runtime/observatory/lib/src/elements/debugger.dart
index f99a2bf..8a98d1b 100644
--- a/runtime/observatory/lib/src/elements/debugger.dart
+++ b/runtime/observatory/lib/src/elements/debugger.dart
@@ -303,7 +303,7 @@
   }
 
   Future run(List<String> args) {
-    debugger.resume();
+    return debugger.resume();
   }
 
   String helpShort = 'Resume execution of the isolate (hotkey: [F7])';