Don't unsubscribe to streams we aren't subscribed to.

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 d7973ea93124882c268e81aeed0fca4ce75fb1c8..f46458120ed20c19723775d3e352f7e3c4945c33 100644
--- a/runtime/observatory/lib/src/elements/debugger.dart
+++ b/runtime/observatory/lib/src/elements/debugger.dart
@@ -1914,6 +1914,7 @@ class DebuggerPageElement extends ObservatoryElement {
       // TODO(turnidge): How do we want to handle this in general?
       _stdoutSubscriptionFuture.catchError((e, st) {
         Logger.root.info('Failed to subscribe to stdout: $e\n$st\n');
+        _stdoutSubscriptionFuture = null;
       });
     }
     _stderrSubscriptionFuture =
@@ -1922,6 +1923,7 @@ class DebuggerPageElement extends ObservatoryElement {
       // TODO(turnidge): How do we want to handle this in general?
       _stderrSubscriptionFuture.catchError((e, st) {
         Logger.root.info('Failed to subscribe to stderr: $e\n$st\n');
+        _stderrSubscriptionFuture = null;
       });
     }
     _logSubscriptionFuture =