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

runtime/lib/array_patch.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.
 
 // The _GrowableArrayMarker class is used to signal to the List() factory
 // whether a parameter was passed.
 class _GrowableArrayMarker implements int {
   const _GrowableArrayMarker();
 }
 
@@ skipping 18 matching lines @@
         result[i] = fill;
       }
     }
     return result;
   }
 
   /* patch */ factory List.from(Iterable elements, { bool growable: true }) {
     if (elements is EfficientLength) {
       int length = elements.length;
       var list = growable ? new _GrowableList<E>(length) : new _List<E>(length);
-      int i = 0;
-      for (var element in elements) { list[i++] = element; }
+      if (length > 0) {  // Avoid creating iterator unless necessary.
+        int i = 0;
+        for (var element in elements) { list[i++] = element; }
+      }
       return list;
     }
     List<E> list = new _GrowableList<E>(0);
     for (E e in elements) {
       list.add(e);
     }
     if (growable) return list;
     if (list.length == 0) {
       // Avoid getting an immutable list from makeListFixedLength.
-      return new List<E>(0);
+      return new _List<E>(0);
     }
     return makeListFixedLength(list);
   }
 
   /* patch */ factory List.unmodifiable(Iterable elements) {
     List result = new List<E>.from(elements, growable: false);
     return makeFixedListUnmodifiable(result);
   }
 
   // Factory constructing a mutable List from a parser generated List literal.
   // [elements] contains elements that are already type checked.
   factory List._fromLiteral(List elements) {
     if (elements.isEmpty) {
       return new _GrowableList<E>(0);
     }
     var result = new _GrowableList<E>.withData(elements);
     result._setLength(elements.length);
     return result;
   }
 }