Add List.unmodifiable constructor.

sdk/lib/_internal/compiler/js_lib/internal_patch.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:_js_primitives' show printString;
 import 'dart:_js_helper' show JS, patch;
 import 'dart:_interceptors' show JSArray;
 
 @patch
 class Symbol implements core.Symbol {
   @patch
   const Symbol(String name)
       : this._name = name;
 }
 
 @patch
 void printToConsole(String line) {
   printString('$line');
 }
 
 @patch
 List makeListFixedLength(List growableList) {
-  JSArray.markFixedList(growableList);
-  return growableList;
+  return JSArray.markFixedList(growableList);
 }
+
+@patch
+List makeFixedListUnmodifiable(List fixedLengthList) {
+  return JSArray.markUnmodifiableList(fixedLengthList);
+}