Use interceptors to handle runtime types.

dart/sdk/lib/_internal/lib/js_names.dart

Index: dart/sdk/lib/_internal/lib/js_names.dart
diff --git a/dart/sdk/lib/_internal/lib/js_names.dart b/dart/sdk/lib/_internal/lib/js_names.dart
index 9eaf777c4119a135644e74d136f77a04511a481b..9232206513962959c5596ee82fc991655ca4b643 100644
--- a/dart/sdk/lib/_internal/lib/js_names.dart
+++ b/dart/sdk/lib/_internal/lib/js_names.dart
@@ -8,6 +8,8 @@ import 'dart:_foreign_helper' show JS, JS_GET_NAME;
 
 import 'dart:_js_helper' show JsCache;
 
+import 'dart:_interceptors' show JSExtendableArray;
+
 /// No-op method that is called to inform the compiler that unmangled named
 /// must be preserved.
 preserveNames() {}
@@ -64,7 +66,7 @@ Map<String, String> computeReflectiveNames(Map<String, String> map) {
 }
 
 List extractKeys(victim) {
-  return JS('List', '''
+  return JS('JSExtendableArray', '''
 (function(victim, hasOwnProperty) {
   var result = [];
   for (var key in victim) {
@@ -87,5 +89,5 @@ List extractKeys(victim) {
  */
 String unmangleGlobalNameIfPreservedAnyways(String name) {
   var names = JS('=Object', 'init.mangledGlobalNames');
-  return JsCache.fetch(names, name);
+  return JS('String|Null', '#', JsCache.fetch(names, name));
 }