Add ApplyChangesStatus to AnalysisContext.applyChanges() and use it in DAS.

pkg/analyzer/test/generated/engine_test.dart

Index: pkg/analyzer/test/generated/engine_test.dart
diff --git a/pkg/analyzer/test/generated/engine_test.dart b/pkg/analyzer/test/generated/engine_test.dart
index 6d04f133b93a74b129b690cb4422d5c01b0bfea1..3f24a4b9ce5557daaf4aacdf8babc88b4e7d9242 100644
--- a/pkg/analyzer/test/generated/engine_test.dart
+++ b/pkg/analyzer/test/generated/engine_test.dart
@@ -5862,8 +5862,9 @@ class TestAnalysisContext implements InternalAnalysisContext {
   }
 
   @override
-  void applyChanges(ChangeSet changeSet) {
+  ApplyChangesStatus applyChanges(ChangeSet changeSet) {
     fail("Unexpected invocation of applyChanges");
+    return null;
   }
 
   @override
@@ -6231,8 +6232,9 @@ class TestAnalysisContext_test_applyChanges extends TestAnalysisContext {
   bool invoked = false;
   TestAnalysisContext_test_applyChanges();
   @override
-  void applyChanges(ChangeSet changeSet) {
+  ApplyChangesStatus applyChanges(ChangeSet changeSet) {
     invoked = true;
+    return new ApplyChangesStatus(false);
   }
 }