Don't cast to ClassElementImpl where we don't need to.

pkg/analyzer/lib/src/generated/element_resolver.dart

Index: pkg/analyzer/lib/src/generated/element_resolver.dart
diff --git a/pkg/analyzer/lib/src/generated/element_resolver.dart b/pkg/analyzer/lib/src/generated/element_resolver.dart
index 06d0c56f3c979dec23e7620b6ac7a52a89b1e8a6..9f0cda01242452fad472655826b09be07e19b09d 100644
--- a/pkg/analyzer/lib/src/generated/element_resolver.dart
+++ b/pkg/analyzer/lib/src/generated/element_resolver.dart
@@ -2129,7 +2129,7 @@ class ElementResolver extends SimpleAstVisitor<Object> {
    * given [propertyName], return the element that represents the property.
    */
   Element _resolveElement(
-      ClassElementImpl classElement, SimpleIdentifier propertyName) {
+      ClassElement classElement, SimpleIdentifier propertyName) {
     String name = propertyName.name;
     Element element = null;
     if (propertyName.inSetterContext()) {