Use IncrementalCompilationUnitElementBuilder in BuildCompilationUnitElementTask.

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

Index: pkg/analyzer/test/src/task/dart_test.dart
diff --git a/pkg/analyzer/test/src/task/dart_test.dart b/pkg/analyzer/test/src/task/dart_test.dart
index c2cfe5c370454c6d7e15d8da9ebc2b587a456296..3d549c5118a08b6a16dc7c77e3048cb3139c3163 100644
--- a/pkg/analyzer/test/src/task/dart_test.dart
+++ b/pkg/analyzer/test/src/task/dart_test.dart
@@ -205,6 +205,26 @@ class B = Object with A;
     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);