diff --git a/pkg/analyzer/lib/src/summary/expr_builder.dart b/pkg/analyzer/lib/src/summary/expr_builder.dart
index bbec013e03a5d..df4be6e1806c6 100644
--- a/pkg/analyzer/lib/src/summary/expr_builder.dart
+++ b/pkg/analyzer/lib/src/summary/expr_builder.dart
@@ -405,6 +405,8 @@ class ExprBuilder {
         return postfix(TokenType.PLUS_PLUS);
       case UnlinkedExprAssignOperator.postfixDecrement:
         return postfix(TokenType.MINUS_MINUS);
+      default:
+        throw new UnimplementedError('Unexpected UnlinkedExprAssignOperator');
     }
   }