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

sdk/lib/internal/list.dart

Index: sdk/lib/internal/list.dart
diff --git a/sdk/lib/internal/list.dart b/sdk/lib/internal/list.dart
index 14eeb1372ee8c050a5bb69528bee6d3e72e9496c..f40c39bffaeee7fa1076ac781f5cd17bca242912 100644
--- a/sdk/lib/internal/list.dart
+++ b/sdk/lib/internal/list.dart
@@ -305,3 +305,29 @@ class ReversedListIterable<E> extends ListIterable<E> {
 
   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);