Get memento objects from tasks, remember in cache and pass back to tasks.

pkg/analyzer/test/src/context/cache_test.dart

Index: pkg/analyzer/test/src/context/cache_test.dart
diff --git a/pkg/analyzer/test/src/context/cache_test.dart b/pkg/analyzer/test/src/context/cache_test.dart
index f635ab9ce908dd2e0863ad3e59c7b0eb66f62dd2..e3cc80469ff9a72549f73c1c89b46a01c21027d3 100644
--- a/pkg/analyzer/test/src/context/cache_test.dart
+++ b/pkg/analyzer/test/src/context/cache_test.dart
@@ -208,6 +208,13 @@ class CacheEntryTest extends EngineTestCase {
     expect(entry.getValue(result), defaultValue);
   }
 
+  test_getMemento_noResult() {
+    String defaultValue = 'value';
+    ResultDescriptor result = new ResultDescriptor('test', defaultValue);
+    CacheEntry entry = new CacheEntry();
+    expect(entry.getMemento(result), null);
+  }
+
   test_hasAstStructure_false() {
     CacheEntry entry = new CacheEntry();
     expect(entry.hasAstStructure, false);
@@ -367,13 +374,15 @@ class CacheEntryTest extends EngineTestCase {
     ResultDescriptor result = new ResultDescriptor('test', 1);
     CacheEntry entry = new CacheEntry();
     // set VALID
-    entry.setValue(result, 10, TargetedResult.EMPTY_LIST);
+    String memento = 'main() {}';
+    entry.setValue(result, 10, TargetedResult.EMPTY_LIST, memento);
     expect(entry.getState(result), CacheState.VALID);
     expect(entry.getValue(result), 10);
     // set INVALID
     entry.setState(result, CacheState.INVALID);
     expect(entry.getState(result), CacheState.INVALID);
     expect(entry.getValue(result), 1);
+    expect(entry.getMemento(result), memento);
   }
 
   test_setState_invalid_invalidateDependent() {
@@ -417,12 +426,14 @@ class CacheEntryTest extends EngineTestCase {
   }
 
   test_setValue() {
-    String value = 'value';
     ResultDescriptor result = new ResultDescriptor('test', null);
+    String value = 'value';
+    String memento = 'main() {}';
     CacheEntry entry = new CacheEntry();
-    entry.setValue(result, value, TargetedResult.EMPTY_LIST);
+    entry.setValue(result, value, TargetedResult.EMPTY_LIST, memento);
     expect(entry.getState(result), CacheState.VALID);
     expect(entry.getValue(result), value);
+    expect(entry.getMemento(result), memento);
   }
 
   test_setValue_invalidateDependent() {