Use new task model in completion tests [TBR].

pkg/analysis_server/test/domain_completion_test.dart

Index: pkg/analysis_server/test/domain_completion_test.dart
diff --git a/pkg/analysis_server/test/domain_completion_test.dart b/pkg/analysis_server/test/domain_completion_test.dart
index b29650832f1e7a386510f9c3d19ad6d62ae5fde0..f0d6f83690428c2519316dc6bc9c1504e2e3c3e7 100644
--- a/pkg/analysis_server/test/domain_completion_test.dart
+++ b/pkg/analysis_server/test/domain_completion_test.dart
@@ -54,6 +54,9 @@ class CompletionManagerTest extends AbstractAnalysisTest {
   int requestCount = 0;
   String testFile2 = '/project/bin/test2.dart';
 
+  /// Cached model state in case tests need to set task model to on/off.
+  bool wasTaskModelEnabled;
+
   AnalysisServer createAnalysisServer(Index index) {
     ExtensionManager manager = new ExtensionManager();
     ServerPlugin serverPlugin = new ServerPlugin();
@@ -84,6 +87,8 @@ class CompletionManagerTest extends AbstractAnalysisTest {
   @override
   void setUp() {
     super.setUp();
+    wasTaskModelEnabled = AnalysisEngine.instance.useTaskModel;
+    AnalysisEngine.instance.useTaskModel = true;
     createProject();
     analysisDomain = handler;
     completionDomain = new Test_CompletionDomainHandler(server);
@@ -96,6 +101,7 @@ class CompletionManagerTest extends AbstractAnalysisTest {
     super.tearDown();
     analysisDomain = null;
     completionDomain = null;
+    AnalysisEngine.instance.useTaskModel = wasTaskModelEnabled;
   }
 
   /**