Use ConstExp for storing constants.

sdk/lib/_internal/compiler/implementation/native_handler.dart

Index: sdk/lib/_internal/compiler/implementation/native_handler.dart
diff --git a/sdk/lib/_internal/compiler/implementation/native_handler.dart b/sdk/lib/_internal/compiler/implementation/native_handler.dart
index 74d3ccf0786a41c00a3cd0a5c4e31b5633100aed..ed80a7f4700dd888580d6e0c9d1d136c19110ec2 100644
--- a/sdk/lib/_internal/compiler/implementation/native_handler.dart
+++ b/sdk/lib/_internal/compiler/implementation/native_handler.dart
@@ -332,7 +332,7 @@ abstract class NativeEnqueuerBase implements NativeEnqueuer {
          !link.isEmpty;
          link = link.tail) {
       MetadataAnnotation annotation = link.head.ensureResolved(compiler);
-      var value = annotation.value;
+      var value = annotation.constant;
       if (value is! ConstructedConstant) continue;
       if (value.type is! InterfaceType) continue;
       if (!identical(value.type.element, annotationClass)) continue;
@@ -1097,7 +1097,7 @@ class NativeBehavior {
          !link.isEmpty;
          link = link.tail) {
       MetadataAnnotation annotation = link.head.ensureResolved(compiler);
-      var value = annotation.value;
+      var value = annotation.constant;
       if (value is! ConstructedConstant) continue;
       if (value.type is! InterfaceType) continue;
       if (!identical(value.type.element, annotationClass)) continue;