Issue 23477. Fix for Quick Fixes in overlay-only files.

pkg/analysis_server/lib/src/analysis_server.dart

Index: pkg/analysis_server/lib/src/analysis_server.dart
diff --git a/pkg/analysis_server/lib/src/analysis_server.dart b/pkg/analysis_server/lib/src/analysis_server.dart
index b101f37bef48dcddd06b4d76c5ac15fe03754fb8..1b7bda309c3fc698102528e7caafc18600a4f6c2 100644
--- a/pkg/analysis_server/lib/src/analysis_server.dart
+++ b/pkg/analysis_server/lib/src/analysis_server.dart
@@ -532,7 +532,7 @@ class AnalysisServer {
     if (context == null) {
       return null;
     }
-    if (!source.exists()) {
+    if (!context.exists(source)) {
       return null;
     }
     return context.getErrors(source);