Optimize List.toList/.sublist and List.from on lists.

runtime/lib/growable_array.dart

 // Copyright (c) 2012, 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.
 
 class _GrowableList<T> implements List<T> {
 
   void insert(int index, T element) {
     if ((index < 0) || (index > length)) {
       throw new RangeError.range(index, 0, length);
     }
@@ skipping 85 matching lines @@
 
   void fillRange(int start, int end, [T fillValue]) {
     IterableMixinWorkaround.fillRangeList(this, start, end, fillValue);
   }
 
   List<T> sublist(int start, [int end]) {
     Lists.indicesCheck(this, start, end);
     if (end == null) end = this.length;
     int length = end - start;
     if (start == end) return <T>[];
-    List list = new _GrowableList<T>.withCapacity(length);
-    list.length = length;
+    List list = new _List(length);
     Lists.copy(this, start, list, 0, length);
-    return list;
+    var result = new _GrowableList<T>.withData(list);
+    result._setLength(length);
+    return result;
   }
 
   static const int _kDefaultCapacity = 2;
 
   factory _GrowableList(int length) {
     var data = new _List((length == 0) ? _kDefaultCapacity : length);
     var result = new _GrowableList<T>.withData(data);
     if (length > 0) {
       result._setLength(length);
     }
@@ skipping 24 matching lines @@
     } else {
       for (int i = new_length; i < length; i++) {
         this[i] = null;
       }
     }
     _setLength(new_length);
   }
 
   void _setLength(int new_length) native "GrowableList_setLength";
 
   void _setData(_List array) native "GrowableList_setData";

[Lasse Reichstein Nielsen, 2014/08/19 12:48:30]

If there was a _getData, we could use _List._copyFromObjectArray to copy into
it. Does that sound like a useful think, or is it too dangeous?

 
   T operator [](int index) native "GrowableList_getIndexed";
 
   void operator []=(int index, T value) native "GrowableList_setIndexed";
 
   // The length of this growable array. It is always less than or equal to the
   // length of the object array, which itself is always greater than 0, so that
   // grow() does not have to check for a zero length object array before
   // doubling its size.
   void add(T value) {
@@ skipping 164 matching lines @@
     IterableMixinWorkaround.shuffleList(this, random);
   }
 
   String toString() => ListBase.listToString(this);
 
   Iterator<T> get iterator {
     return new ListIterator<T>(this);
   }
 
   List<T> toList({ bool growable: true }) {
-    return new List<T>.from(this, growable: growable);
+    var length = this.length;
+    var result = growable ? new _GrowableList<T>(length) : new _List<T>(length);
+    Lists.copy(this, 0, result, 0, length);
+    return result;
   }
 
   Set<T> toSet() {
     return new Set<T>.from(this);
   }
 
   Map<int, T> asMap() {
     return new IterableMixinWorkaround<T>().asMapList(this);
   }
 }