Fix incremental resolution tests

pkg/analyzer/test/generated/incremental_resolver_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 engine.incremental_resolver_test;
 
 import 'package:analyzer/src/context/cache.dart' as task;
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/engine.dart';
@@ skipping 4460 matching lines @@
     // The existing CompilationUnitElement should be updated.
     expect(newUnit.element, same(oldUnitElement));
     // The only expected pending task should return the same resolved
     // "newUnit", so all clients will get it using the usual way.
     AnalysisResult analysisResult = analysisContext.performAnalysisTask();
     ChangeNotice notice = analysisResult.changeNotices[0];
     expect(notice.resolvedDartUnit, same(newUnit));
     // Resolve "newCode" from scratch.
     if (compareWithFull) {
       _resetWithIncremental(false);
-      source = addSource(newCode + ' ');
-      source = addSource(newCode);
+      changeSource(source, newCode);
       _runTasks();
       LibraryElement library = resolve2(source);
       CompilationUnit fullNewUnit = resolveCompilationUnit(source, library);
       // Validate tokens.
       _assertEqualTokens(newUnit, fullNewUnit);
       // Validate LineInfo
       _assertEqualLineInfo(newLineInfo, analysisContext.getLineInfo(source));
       // Validate that "incremental" and "full" units have the same resolution.
       try {
         assertSameResolution(newUnit, fullNewUnit, validateTypes: true);
@@ skipping 250 matching lines @@
     return ResolutionContextBuilder.contextFor(node, listener).scope;
   }
 }
 
 class _Edit {
   final int offset;
   final int length;
   final String replacement;
   _Edit(this.offset, this.length, this.replacement);
 }