Fix a bug that caused a runtime error when attempting to read metadata of a class without metadata.

dart/sdk/lib/_internal/lib/js_mirrors.dart

Index: dart/sdk/lib/_internal/lib/js_mirrors.dart
diff --git a/dart/sdk/lib/_internal/lib/js_mirrors.dart b/dart/sdk/lib/_internal/lib/js_mirrors.dart
index 86193b20aaa0930e9175f57406308fac301aaa97..a1c40bf8161fe3b51deda2ee36080761b9352be7 100644
--- a/dart/sdk/lib/_internal/lib/js_mirrors.dart
+++ b/dart/sdk/lib/_internal/lib/js_mirrors.dart
@@ -1125,6 +1125,7 @@ List extractMetadata(victim) {
   preserveMetadata();
   var metadataFunction = JS('', '#["@"]', victim);
   if (metadataFunction != null) return JS('', '#()', metadataFunction);
+  if (JS('String', 'typeof #', victim) != 'function') return const [];
   String source = JS('String', 'Function.prototype.toString.call(#)', victim);
   int index = source.lastIndexOf(new RegExp('"[0-9,]*";?[ \n\r]*}'));
   if (index == -1) return const [];