Add WhereIterable.map

This makes some nice code improvements for dart2js. dart2js can't
inline the MappedIterable factory constructor and specialize by
optimization, so specializing by hand removes the test "is
EfficientLength" for code like "x.where(f).map(g)".

R=lrn@google.com

Review URL: https://codereview.chromium.org/2354093002 .

sdk/lib/internal/iterable.dart

@@ -426,6 +426,10 @@ class WhereIterable<E> extends Iterable<E> {
   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);
 }
 
 class WhereIterator<E> extends Iterator<E> {