[VM] Fix for issue 34839 Service isolate hangs during shutdown if

     the VM exits before the Observatory server has a chance to
     start listening.

Bug: 34839
Change-Id: I2a5dfe69a197fe054dd041af3ea4e07da736b226
Reviewed-on: https://dart-review.googlesource.com/c/81381
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Siva Annamalai <asiva@google.com>

runtime/bin/vmservice/server.dart

@@ -373,6 +373,12 @@ class Server {
       }
       await new Future<Null>.delayed(const Duration(seconds: 1));
     }
+    if (_service.isExiting) {
+      serverPrint('Observatory HTTP server exiting before listening as '
+          'vm service has received exit request\n');
+      await shutdown(true);
+      return this;
+    }
     _server.listen(_requestHandler, cancelOnError: true);
     serverPrint('Observatory listening on $serverAddress');
     if (Platform.isFuchsia) {

sdk/lib/vmservice/vmservice.dart

@@ -204,6 +204,9 @@ class VMService extends MessageRouter {
   /// Collection of currently running isolates.
   RunningIsolates runningIsolates = new RunningIsolates();
 
+  /// Flag to indicate VM service is exiting.
+  bool isExiting = false;
+
   /// A port used to receive events from the VM.
   final RawReceivePort eventPort;
 
@@ -323,6 +326,8 @@ class VMService extends MessageRouter {
   }
 
   Future _exit() async {
+    isExiting = true;
+
     // Stop the server.
     if (VMServiceEmbedderHooks.serverStop != null) {
       await VMServiceEmbedderHooks.serverStop();
@@ -342,6 +347,7 @@ class VMService extends MessageRouter {
     if (VMServiceEmbedderHooks.cleanup != null) {
       await VMServiceEmbedderHooks.cleanup();
     }
+
     // Notify the VM that we have exited.
     _onExit();
   }