Issue 26106. Don't attempt to incrementally resolve sources without contents.

pkg/analyzer/lib/src/context/context.dart

Index: pkg/analyzer/lib/src/context/context.dart
diff --git a/pkg/analyzer/lib/src/context/context.dart b/pkg/analyzer/lib/src/context/context.dart
index bef1653e8ec325df92817dfb1479fd4aa5faa73f..d2659baf50c4ab302d1c92cf233bc72c992fd65e 100644
--- a/pkg/analyzer/lib/src/context/context.dart
+++ b/pkg/analyzer/lib/src/context/context.dart
@@ -1066,7 +1066,8 @@ class AnalysisContextImpl implements InternalAnalysisContext {
       // If not the same content (e.g. the file is being closed without save),
       // then force analysis.
       if (changed) {
-        if (!analysisOptions.incremental ||
+        if (newContents == null ||
+            !analysisOptions.incremental ||
             !_tryPoorMansIncrementalResolution(source, newContents)) {
           _sourceChanged(source);
         }