Initial version of limiting invalidation.

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

Index: pkg/analyzer/lib/src/task/incremental_element_builder.dart
diff --git a/pkg/analyzer/lib/src/task/incremental_element_builder.dart b/pkg/analyzer/lib/src/task/incremental_element_builder.dart
index 4896b5d64ad13feb8afe74fea95a1ed77d60f951..2e4a5366a9458b27f5af16b9a5ad480716a1ae8e 100644
--- a/pkg/analyzer/lib/src/task/incremental_element_builder.dart
+++ b/pkg/analyzer/lib/src/task/incremental_element_builder.dart
@@ -214,8 +214,13 @@ class IncrementalCompilationUnitElementBuilder {
         for (VariableDeclaration variable in variableList.variables) {
           TopLevelVariableElement element = variable.element;
           elements.add(element);
-          elements.add(element.getter);
-          elements.add(element.setter);
+          // TODO(scheglov) write tests for nulls
+          if (element.getter != null) {
+            elements.add(element.getter);
+          }
+          if (element.setter != null) {
+            elements.add(element.setter);
+          }
         }
       }
     } else if (node is PartDirective || node is PartOfDirective) {