Use IncrementalCompilationUnitElementBuilder in BuildCompilationUnitElementTask.

pkg/analyzer/test/src/task/dart_test.dart

 // Copyright (c) 2015, 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.src.task.dart_test;
 
 import 'package:analyzer/src/context/cache.dart';
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/error.dart';
@@ skipping 187 matching lines @@
 part 'part.dart';
 class A {}
 class B = Object with A;
 ''');
     expect(outputs, hasLength(3));
     expect(outputs[CLASS_ELEMENTS], hasLength(2));
     expect(outputs[COMPILATION_UNIT_ELEMENT], isNotNull);
     expect(outputs[RESOLVED_UNIT1], isNotNull);
   }
 
+  test_perform_useMemento() {
+    String content = r'''
+library lib;
+import 'lib2.dart';
+export 'lib3.dart';
+part 'part.dart';
+class A {''';
+    Source source = newSource('/test.dart', content);
+    AnalysisTarget target = new LibrarySpecificUnit(source, source);
+    _computeResult(target, RESOLVED_UNIT1);
+    // update content
+    context.setContents(source, content);
+    assertIsInvalid(target, RESOLVED_UNIT1);
+    // recompute
+    _computeResult(target, RESOLVED_UNIT1);
+    assertIsValid(target, RESOLVED_UNIT1);
+    // values from the memento are returned
+    assertSameResults(ParseDartTask.DESCRIPTOR.results);
+  }
+
   void _performBuildTask(String content) {
     Source source = newSource('/test.dart', content);
     AnalysisTarget target = new LibrarySpecificUnit(source, source);
     _computeResult(target, RESOLVED_UNIT1);
     expect(task, new isInstanceOf<BuildCompilationUnitElementTask>());
   }
 }
 
 @reflectiveTest
 class BuildDirectiveElementsTaskTest extends _AbstractDartTaskTest {
@@ skipping 1688 matching lines @@
   /**
    * Fill [errorListener] with [result] errors in the current [task].
    */
   void _fillErrorListener(ResultDescriptor<List<AnalysisError>> result) {
     List<AnalysisError> errors = task.outputs[result];
     expect(errors, isNotNull, reason: result.name);
     errorListener = new GatheringErrorListener();
     errorListener.addAll(errors);
   }
 }