Add internal operation that converts a growable list to a fixed list.

sdk/lib/_internal/lib/internal_patch.dart

Index: sdk/lib/_internal/lib/internal_patch.dart
diff --git a/sdk/lib/_internal/lib/internal_patch.dart b/sdk/lib/_internal/lib/internal_patch.dart
index cccc6569f5e4db64e692815f57fc58ddc89c7bef..485c8956de4fc8e34a10d138e0906fc50f142893 100644
--- a/sdk/lib/_internal/lib/internal_patch.dart
+++ b/sdk/lib/_internal/lib/internal_patch.dart
@@ -3,6 +3,8 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:_js_primitives' show printString;
+import 'dart:_js_helper' show JS;
+import 'dart:_interceptors' show JSArray;
 
 patch class Symbol implements core.Symbol {
   patch const Symbol(String name)
@@ -11,4 +13,9 @@ patch class Symbol implements core.Symbol {
 
 patch void printToConsole(String line) {
   printString('$line');
-}
+}
+
+patch List makeListFixedLength(List growableList) {
+  JSArray.markFixedList(growableList);
+  return growableList;
+}