gracefully degrade with response to client if fail to access client specified dart sdk

pkg/analysis_server/lib/src/analysis_manager.dart

Index: pkg/analysis_server/lib/src/analysis_manager.dart
diff --git a/pkg/analysis_server/lib/src/analysis_manager.dart b/pkg/analysis_server/lib/src/analysis_manager.dart
index 00602e4183b096714e03d7a4420d3ddf1d6ff9d9..ce7f787992e532d88c8c88fe533eacfece581c54 100644
--- a/pkg/analysis_server/lib/src/analysis_manager.dart
+++ b/pkg/analysis_server/lib/src/analysis_manager.dart
@@ -126,8 +126,7 @@ class AnalysisManager {
           print('Expected shutdown response');
         })
         .then((Response response) {
-          channel.close();
-          return process.exitCode;
+          return channel.close().then((_) => process.exitCode);
         })
         .timeout(new Duration(seconds: 2), onTimeout: () {
           print('Expected server to shutdown');