Add only dart files to drivers.

Change-Id: I345d3481a9107b6eec1e7e8845e8181f9d12a1ed
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/186500
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
diff --git a/pkg/analysis_server/lib/src/context_manager.dart b/pkg/analysis_server/lib/src/context_manager.dart
index bdbede3..0de59b9 100644
--- a/pkg/analysis_server/lib/src/context_manager.dart
+++ b/pkg/analysis_server/lib/src/context_manager.dart
@@ -439,7 +439,9 @@
         if (_isContainedInDotFolder(contextImpl.contextRoot.root.path, file)) {
           continue;
         }
-        driver.addFile(file);
+        if (file_paths.isDart(pathContext, file)) {
+          driver.addFile(file);
+        }
       }
 
       var optionsFile = context.contextRoot.optionsFile;