Issue 26355. Don't overwrite full index with declarations-only one.

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

 // Copyright (c) 2016, 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.
 
 import 'package:analysis_server/src/services/index/index.dart';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/summary/idl.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
@@ skipping 230 matching lines @@
 ''');
     List<Location> locations =
         await index.getUnresolvedMemberReferences('test');
     expect(locations, hasLength(4));
     findLocationTest(locations, 'test);', false);
     findLocationTest(locations, 'test = 1;', false);
     findLocationTest(locations, 'test += 2;', false);
     findLocationTest(locations, 'test();', false);
   }
 
+  test_indexDeclarations_afterIndexUnit() async {
+    resolveTestUnit('''
+var a = 0;
+var b = a + 1;
+''');
+    index.indexUnit(testUnit);
+    TopLevelVariableElement a = findElement('a');
+    // We can find references.
+    {
+      List<Location> locations = await index.getRelations(
+          a.getter, IndexRelationKind.IS_REFERENCED_BY);
+      findLocationTest(locations, 'a + 1', false);
+    }
+    // Attempt to index just declarations - we still can find references.
+    index.indexDeclarations(testUnit);
+    {
+      List<Location> locations = await index.getRelations(
+          a.getter, IndexRelationKind.IS_REFERENCED_BY);
+      findLocationTest(locations, 'a + 1', false);
+    }
+  }
+
   test_indexDeclarations_nullUnit() async {
     index.indexDeclarations(null);
   }
 
   test_indexDeclarations_nullUnitElement() async {
     resolveTestUnit('');
     testUnit.element = null;
     index.indexDeclarations(testUnit);
   }
 
@@ skipping 85 matching lines @@
   Source _indexUnit(String path, String code) {
     Source source = addSource(path, code);
     CompilationUnit unit = resolveLibraryUnit(source);
     index.indexUnit(unit);
     return source;
   }
 }
 
 class _CompilationUnitElementMock extends TypedMock
     implements CompilationUnitElement {}