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

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

Index: pkg/analyzer/lib/src/dart/element/type.dart
diff --git a/pkg/analyzer/lib/src/dart/element/type.dart b/pkg/analyzer/lib/src/dart/element/type.dart
index 4268f1ce5ccdbba8bd20f95e3644d7f8f168d5e1..6b587cd5c378b37c5b9ccc6f198af6d4510c4b85 100644
--- a/pkg/analyzer/lib/src/dart/element/type.dart
+++ b/pkg/analyzer/lib/src/dart/element/type.dart
@@ -1295,16 +1295,16 @@ class InterfaceTypeImpl extends TypeImpl implements InterfaceType {
   }
 
   @override
-  PropertyAccessorElement getGetter(String getterName) => PropertyAccessorMember
-      .from((element as ClassElementImpl).getGetter(getterName), this);
+  PropertyAccessorElement getGetter(String getterName) =>
+      PropertyAccessorMember.from(element.getGetter(getterName), this);
 
   @override
-  MethodElement getMethod(String methodName) => MethodMember.from(
-      (element as ClassElementImpl).getMethod(methodName), this);
+  MethodElement getMethod(String methodName) =>
+      MethodMember.from(element.getMethod(methodName), this);
 
   @override
-  PropertyAccessorElement getSetter(String setterName) => PropertyAccessorMember
-      .from((element as ClassElementImpl).getSetter(setterName), this);
+  PropertyAccessorElement getSetter(String setterName) =>
+      PropertyAccessorMember.from(element.getSetter(setterName), this);
 
   @override
   bool isDirectSupertypeOf(InterfaceType type) {