Keep known valid Dart results and invalidate all the other results during incremental resolution.

pkg/analyzer/test/src/context/context_test.dart

Index: pkg/analyzer/test/src/context/context_test.dart
diff --git a/pkg/analyzer/test/src/context/context_test.dart b/pkg/analyzer/test/src/context/context_test.dart
index bad2cc644756ea7b755bc56f93be7f3fe6a41f15..acfb596b846f43f5bf5d8cab4af6ce24492aedd6 100644
--- a/pkg/analyzer/test/src/context/context_test.dart
+++ b/pkg/analyzer/test/src/context/context_test.dart
@@ -2239,7 +2239,7 @@ class B {
 }
 ''');
     _assertInvalid(sourceA, LIBRARY_ERRORS_READY);
-    _assertValid(sourceB, LIBRARY_ERRORS_READY);
+    _assertValid(sourceB, LIBRARY_ELEMENT);
     // The a.dart's unit and element are updated incrementally.
     // They are the same instances as initially.
     // So, all the references from other units are still valid.
@@ -2275,7 +2275,7 @@ class B {
 }
 ''');
     _assertInvalid(sourceA, LIBRARY_ERRORS_READY);
-    _assertInvalid(sourceB, LIBRARY_ERRORS_READY);
+    _assertInvalid(sourceB, LIBRARY_ELEMENT);
     // The a.dart's unit and element are the same.
     {
       LibrarySpecificUnit target = new LibrarySpecificUnit(sourceA, sourceA);