fix warning in noSuchMethod registry

pkg/compiler/lib/src/js_backend/no_such_method_registry.dart

Index: pkg/compiler/lib/src/js_backend/no_such_method_registry.dart
diff --git a/pkg/compiler/lib/src/js_backend/no_such_method_registry.dart b/pkg/compiler/lib/src/js_backend/no_such_method_registry.dart
index bc57616870e054bebb40dad00c2c7b24b2f4b8a1..678f805a9edb0f35a0afd589d6bf3a765fe33225 100644
--- a/pkg/compiler/lib/src/js_backend/no_such_method_registry.dart
+++ b/pkg/compiler/lib/src/js_backend/no_such_method_registry.dart
@@ -208,8 +208,9 @@ class NoSuchMethodRegistry {
     }
     if (expr is Send &&
         expr.isTypeCast &&
+        expr.typeAnnotationFromIsCheckOrCast.typeName is Identifier &&
         expr.typeAnnotationFromIsCheckOrCast.typeName.source == "dynamic") {
-      expr = expr.receiver;
+      expr = (expr as Send).receiver;
     }
     if (expr is Send &&
         expr.isSuperCall &&