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

sdk/lib/internal/list.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.
 
 part of dart._internal;
 
 /**
  * Mixin that throws on the length changing operations of [List].
  *
  * Intended to mix-in on top of [ListMixin] for fixed-length lists.
@@ skipping 287 matching lines @@
 }
 
 class ReversedListIterable<E> extends ListIterable<E> {
   Iterable<E> _source;
   ReversedListIterable(this._source);
 
   int get length => _source.length;
 
   E elementAt(int index) => _source.elementAt(_source.length - 1 - index);
 }
+
+/**
+ * Converts a growable list to a fixed length list with the same elements.
+ *
+ * For internal use only.
+ * Only works on growable lists as created by `[]` or `new List()`.
+ * May throw on any other list.
+ *
+ * The operation is efficient. It doesn't copy the elements, but converts
+ * the existing list directly to a fixed length list.
+ * That means that it is a destructive conversion.
+ * The original list should not be used afterwards.
+ *
+ * The returned list may be the same list as the orginal,
+ * or it may be a different list (according to [identical]).
+ * The original list may have changed type to be a fixed list,
+ * or become empty or been otherwise modified.
+ * It will still be a valid object, so references to it will not, e.g., crash
+ * the runtime if accessed, but no promises are made wrt. its contents.
+ *
+ * This unspecified behavior is the reason the function is not exposed to
+ * users. We allow the underlying implementation to make the most efficient
+ * conversion, at the cost of leaving the original list in an unspecified
+ * state.
+ */
+external List makeListFixedLength(List growableList);