Create synthetic FieldElement(s) (don't use non-synthetic ones) for non-synthetic class getters/set…

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 5d8496d6527da45c4cfc3eae964ff4101300592e..ee428bd96e98c30c1e5978f4ffd4ae56964320e4 100644
--- a/pkg/analyzer/lib/src/task/incremental_element_builder.dart
+++ b/pkg/analyzer/lib/src/task/incremental_element_builder.dart
@@ -214,9 +214,9 @@ class IncrementalCompilationUnitElementBuilder {
     ClassElementDelta classDelta =
         new ClassElementDelta(classElement, librarySource, classElement.name);
     // Prepare all old member elements.
-    var removedAccessors = new Set<PropertyAccessorElement>();
-    var removedConstructors = new Set<ConstructorElement>();
-    var removedMethods = new Set<MethodElement>();
+    var removedAccessors = new Set<PropertyAccessorElement>.identity();
+    var removedConstructors = new Set<ConstructorElement>.identity();
+    var removedMethods = new Set<MethodElement>.identity();
     removedAccessors.addAll(classElement.accessors);
     removedConstructors.addAll(classElement.constructors);
     removedMethods.addAll(classElement.methods);