Fix missing_return warning in expr_builder.dart

Change-Id: Ia1157d668f46114ded9f017dce3b11e202c6a76b
Reviewed-on: https://dart-review.googlesource.com/48561
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

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');
     }
   }