Add a task to the new task model for computing constant values.

pkg/analyzer/lib/src/plugin/engine_plugin.dart

Index: pkg/analyzer/lib/src/plugin/engine_plugin.dart
diff --git a/pkg/analyzer/lib/src/plugin/engine_plugin.dart b/pkg/analyzer/lib/src/plugin/engine_plugin.dart
index ed220af21ba541ffa6e46b14e0aa5b6ee11cd350..bb1a4b8c90104cac62d9d884e565e4b055592811 100644
--- a/pkg/analyzer/lib/src/plugin/engine_plugin.dart
+++ b/pkg/analyzer/lib/src/plugin/engine_plugin.dart
@@ -73,6 +73,7 @@ class EnginePlugin implements Plugin {
     registerExtension(taskId, BuildSourceClosuresTask.DESCRIPTOR);
     registerExtension(taskId, BuildTypeProviderTask.DESCRIPTOR);
     registerExtension(taskId, ComputeConstantDependenciesTask.DESCRIPTOR);
+    registerExtension(taskId, ComputeConstantValueTask.DESCRIPTOR);
     registerExtension(taskId, ContainingLibrariesTask.DESCRIPTOR);
     registerExtension(taskId, DartErrorsTask.DESCRIPTOR);
     registerExtension(taskId, GatherUsedImportedElementsTask.DESCRIPTOR);