Fix lints: unnecessary_brace_in_string_interp

pkg/analysis_server/test/services/index/dart_index_contributor_test.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // 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.
 
 library test.services.src.index.dart_index_contributor;
 
 import 'package:analysis_server/src/provisional/index/index_core.dart';
 import 'package:analysis_server/src/services/index/index.dart';
 import 'package:analysis_server/src/services/index/index_contributor.dart';
 import 'package:analysis_server/src/services/index/index_store.dart';
@@ skipping 1647 matching lines @@
   }
 
   /**
    * Asserts that [recordedRelations] has no item with the specified properties.
    */
   void _assertNoRecordedRelation(IndexableObject expectedIndexable,
       RelationshipImpl relationship, ExpectedLocation location) {
     for (RecordedRelation recordedRelation in recordedRelations) {
       if (_equalsRecordedRelation(
           recordedRelation, expectedIndexable, relationship, location)) {
-        fail('not expected: ${recordedRelation} in\n' +
+        fail('not expected: $recordedRelation in\n' +
             recordedRelations.join('\n'));
       }
     }
   }
 
   /**
    * Asserts that [recordedRelations] has no item with the specified properties.
    */
   void _assertNoRecordedRelationForElement(Element expectedElement,
       RelationshipImpl relationship, ExpectedLocation location) {
@@ skipping 115 matching lines @@
   RecordedRelation(this.indexable, this.relationship, this.location);
 
   @override
   String toString() {
     return 'RecordedRelation(indexable=$indexable; relationship=$relationship; '
         'location=$location; flags='
         '${location.isQualified ? "Q" : ""}'
         '${location.isResolved ? "R" : ""})';
   }
 }