Use entities in the TypeMask interface.

pkg/compiler/lib/src/inferrer/type_graph_nodes.dart

Index: pkg/compiler/lib/src/inferrer/type_graph_nodes.dart
diff --git a/pkg/compiler/lib/src/inferrer/type_graph_nodes.dart b/pkg/compiler/lib/src/inferrer/type_graph_nodes.dart
index fe98524488a7c470774e773e93774a83aa135973..8ad5da6bf4e417731f96bf9bdb32ea08ffbeb490 100644
--- a/pkg/compiler/lib/src/inferrer/type_graph_nodes.dart
+++ b/pkg/compiler/lib/src/inferrer/type_graph_nodes.dart
@@ -11,7 +11,7 @@ import '../common/names.dart' show Identifiers;
 import '../compiler.dart' show Compiler;
 import '../constants/values.dart';
 import '../elements/resolution_types.dart'
-    show DartType, FunctionType, TypeKind;
+    show DartType, FunctionType, InterfaceType, TypeKind;
 import '../elements/elements.dart';
 import '../js_backend/backend.dart';
 import '../tree/dartstring.dart' show DartString;
@@ -1739,8 +1739,8 @@ TypeMask _narrowType(
   } else if (annotation.isVoid) {
     otherType = closedWorld.commonMasks.nullType;
   } else {
-    assert(annotation.isInterfaceType);
-    otherType = new TypeMask.nonNullSubtype(annotation.element, closedWorld);
+    InterfaceType interfaceType = annotation;
+    otherType = new TypeMask.nonNullSubtype(interfaceType.element, closedWorld);
   }
   if (isNullable) otherType = otherType.nullable();
   if (type == null) return otherType;