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

pkg/analyzer/lib/src/dart/element/builder.dart

Index: pkg/analyzer/lib/src/dart/element/builder.dart
diff --git a/pkg/analyzer/lib/src/dart/element/builder.dart b/pkg/analyzer/lib/src/dart/element/builder.dart
index 3b09bef1124ef44017a358ce4f1c2baa5890de48..6eaaedb6910bc133bddad43cea1e41ba51b3b927 100644
--- a/pkg/analyzer/lib/src/dart/element/builder.dart
+++ b/pkg/analyzer/lib/src/dart/element/builder.dart
@@ -976,7 +976,7 @@ class ElementBuilder extends RecursiveAstVisitor<Object> {
         SimpleIdentifier propertyNameNode = node.name;
         String propertyName = propertyNameNode.name;
         FieldElementImpl field =
-            _currentHolder.getField(propertyName) as FieldElementImpl;
+            _currentHolder.getField(propertyName, synthetic: true) as FieldElementImpl;
         if (field == null) {
           field = new FieldElementImpl(node.name.name, -1);
           field.final2 = true;