Use textual expectations for SelectionTest.

Currently the checks that we have are ironically both repetitive and
inconsistent. There are also tests that are not precise / demonstrative.

Change-Id: I1ad0de5b9fb982d8091e4f80a6e5c5ebbad0f418
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/308580
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>

pkg/analysis_server/test/services/completion/dart/text_expectations.dart

@@ -13,6 +13,7 @@ class TextExpectationsCollector {
 
   static const assertMethods = {
     'AbstractCompletionDriverTest.assertResponse',
+    'SelectionTest._assertSelection',
   };
 
   static final Map<String, _File> _files = {};