Add WhereIterable.map

sdk/lib/internal/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._internal;
 
 /**
  * Marker interface for [Iterable] subclasses that have an efficient
  * [length] implementation.
  */
@@ skipping 408 matching lines @@
 
 typedef bool _ElementPredicate<E>(E element);
 
 class WhereIterable<E> extends Iterable<E> {
   final Iterable<E> _iterable;
   final _ElementPredicate<E> _f;
 
   WhereIterable(this._iterable, bool this._f(E element));
 
   Iterator<E> get iterator => new WhereIterator<E>(_iterable.iterator, _f);
+
+  // Specialization of [Iterable.map] to non-EfficientLength.
+  Iterable/*<T>*/ map/*<T>*/(/*=T*/ f(E element)) =>
+     new MappedIterable<E, dynamic/*=T*/>._(this, f);

[Lasse Reichstein Nielsen, 2016/09/21 07:17:13]

I don't see why this makes any difference - the method is completely identical
to the one inherited from Iterable?
There is nothing about EfficientLength in the Iterable version either.

[sra1, 2016/09/21 16:26:22]

The difference is
new MappedIterable<E, T>(this, f)
new MappedIterable<E, T>._(this, f)

The former has the expensive type test and two classes in the return type, and
cannot be inlined by dart2js (too big)
The latter has no type test and a single class in the return type which enables
further optimizations and/or inlining

[Lasse Reichstein Nielsen, 2016/09/21 19:42:16]

Ah, I completely missed that difference :)

 }
 
 class WhereIterator<E> extends Iterator<E> {
   final Iterator<E> _iterator;
   final _ElementPredicate _f;
 
   WhereIterator(this._iterator, bool this._f(E element));
 
   bool moveNext() {
     while (_iterator.moveNext()) {
@@ skipping 319 matching lines @@
  * Creates errors throw by [Iterable] when the element count is wrong.
  */
 abstract class IterableElementError {
   /** Error thrown thrown by, e.g., [Iterable.first] when there is no result. */
   static StateError noElement() => new StateError("No element");
   /** Error thrown by, e.g., [Iterable.single] if there are too many results. */
   static StateError tooMany() => new StateError("Too many elements");
   /** Error thrown by, e.g., [List.setRange] if there are too few elements. */
   static StateError tooFew() => new StateError("Too few elements");
 }