Migrate tests using evaluate_common to null-safety (#1714)

Author: elliette

dwds/lib/src/services/expression_compiler_service.dart

@@ -188,7 +188,7 @@ class _Compiler {
     final error = (errors != null && errors.isNotEmpty)
         ? errors.first
         : (e != null ? '$e:$s' : '<unknown error>');
-    final procedure = response['compiledProcedure'] as String;
+    final procedure = (response['compiledProcedure'] as String?) ?? '';
     final succeeded = (response['succeeded'] as bool?) ?? false;
     final result = succeeded ? procedure : error;
 

dwds/test/build_daemon_evaluate_test.dart

@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 @TestOn('vm')
 
 import 'package:test/test.dart';