Issue 26044. Cache and use LibraryElementImpl.exportNamespace.

pkg/analyzer/lib/src/task/dart.dart

Index: pkg/analyzer/lib/src/task/dart.dart
diff --git a/pkg/analyzer/lib/src/task/dart.dart b/pkg/analyzer/lib/src/task/dart.dart
index 063a7d9ab2793a40ac1a328dd4c0d97c936345d7..accc28ab856fcca000a86fecc8852105c540a7ed 100644
--- a/pkg/analyzer/lib/src/task/dart.dart
+++ b/pkg/analyzer/lib/src/task/dart.dart
@@ -2231,7 +2231,7 @@ class DartDelta extends Delta {
 
   @override
   DeltaResult validate(InternalAnalysisContext context, AnalysisTarget target,
-      ResultDescriptor descriptor) {
+      ResultDescriptor descriptor, CacheState state, Object value) {
     if (hasDirectiveChange) {
       return DeltaResult.INVALIDATE;
     }
@@ -2259,6 +2259,10 @@ class DartDelta extends Delta {
         return DeltaResult.KEEP_CONTINUE;
       }
       if (BuildLibraryElementTask.DESCRIPTOR.results.contains(descriptor)) {
+        // Invalidate cached results.
+        if (value is LibraryElementImpl) {
+          value.exportNamespace = null;
+        }
         return DeltaResult.KEEP_CONTINUE;
       }
       return DeltaResult.INVALIDATE;