Make Iterable.toList more efficient if the length is known.

sdk/lib/core/iterable.dart

 // Copyright (c) 2011, 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.core;
 
 /**
  * A collection of values, or "elements", that can be accessed sequentially.
  *
  * The elements of the iterable are accessed by getting an [Iterator]
@@ skipping 318 matching lines @@
     }
     return false;
   }
 
   /**
    * Creates a [List] containing the elements of this [Iterable].
    *
    * The elements are in iteration order.
    * The list is fixed-length if [growable] is false.
    */
-  List<E> toList({ bool growable: true }) =>
-      new List<E>.from(this, growable: growable);
+  List<E> toList({ bool growable: true }) {
+    if (this is! EfficientLength) {

[floitsch, 2016/05/20 12:46:02]

This check is already done in List.from.

+      return new List<E>.from(this, growable: growable);
+    }
+    return _toListKnownLength(this.length, growable);
+  }
+
+  /// Converts this Iterable to a list with the given length.
+  List<E> _toListKnownLength(int length, bool growable) {
+    List<E> result;
+    if (growable) {
+      result = new List<E>()..length = length;
+    } else {
+      result = new List<E>(length);
+    }
+    int i = 0;
+    for (var element in this) {
+      result[i++] = element;
+    }
+    if (i != length) {
+      throw new ConcurrentModificationError(this);
+    }
+    return result;
+  }
 
   /**
    * Creates a [Set] containing the same elements as this iterable.
    *
    * The set may contain fewer elements than the iterable,
    * if the iterable contains an element more than once,
    * or it contains one or more elements that are equal.
    * The order of the elements in the set is not guaranteed to be the same
    * as for the iterable.
    */
@@ skipping 286 matching lines @@
  */
 abstract class BidirectionalIterator<E> implements Iterator<E> {
   /**
    * Move back to the previous element.
    *
    * Returns true and updates [current] if successful. Returns false
    * and sets [current] to null if there is no previous element.
    */
   bool movePrevious();
 }