Use memento in ScanDartTask.

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 ddabeb38ce0b4e6e507f93bb7144a91c61e08e98..0bc79c49503c3dfd6b56f71d0362c44bbe2baeca 100644
--- a/pkg/analyzer/test/src/task/dart_test.dart
+++ b/pkg/analyzer/test/src/task/dart_test.dart
@@ -1756,6 +1756,14 @@ main(p1, p2, p3, p4) {
 
 @reflectiveTest
 class ScanDartTaskTest extends _AbstractDartTaskTest {
+  void assertSameResults(List<ResultDescriptor> descriptors) {
+    descriptors.forEach((descriptor) {
+      var oldResult = oldOutputs[descriptor];
+      var newResult = outputs[descriptor];
+      expect(newResult, same(oldResult));
+    });
+  }
+
   test_buildInputs() {
     Map<String, TaskInput> inputs = ScanDartTask.buildInputs(emptySource);
     expect(inputs, isNotNull);
@@ -1802,6 +1810,17 @@ class ScanDartTaskTest extends _AbstractDartTaskTest {
     expect(outputs[TOKEN_STREAM], isNotNull);
   }
 
+  test_perform_useMemento() {
+    AnalysisTarget target = newSource('/test.dart', 'main() {}');
+    _computeResult(target, TOKEN_STREAM);
+    // update content
+    context.setContents(target, 'main() {}');
+    // recompute
+    _computeResult(target, TOKEN_STREAM);
+    // values from the mememto are returned
+    assertSameResults([LINE_INFO, SCAN_ERRORS, TOKEN_STREAM]);
+  }
+
   void _performScanTask(String content) {
     AnalysisTarget target = newSource('/test.dart', content);
     _computeResult(target, TOKEN_STREAM);
@@ -1832,6 +1851,7 @@ main() {
 class _AbstractDartTaskTest extends AbstractContextTest {
   Source emptySource;
 
+  Map<ResultDescriptor<dynamic>, dynamic> oldOutputs;
   AnalysisTask task;
   Map<ResultDescriptor<dynamic>, dynamic> outputs;
   GatheringErrorListener errorListener = new GatheringErrorListener();
@@ -1842,6 +1862,7 @@ class _AbstractDartTaskTest extends AbstractContextTest {
   }
 
   void _computeResult(AnalysisTarget target, ResultDescriptor result) {
+    oldOutputs = outputs;
     task = analysisDriver.computeResult(target, result);
     expect(task.caughtException, isNull);
     outputs = task.outputs;