Make 'where' lazy.

sdk/lib/core/sequences.dart

Index: sdk/lib/core/sequences.dart
diff --git a/sdk/lib/core/sequences.dart b/sdk/lib/core/sequences.dart
index a98fa8dd19838c3c570515a0822ed2bf348557b6..b20f673acb03d13329c6e5fbc0f1528664bba3bb 100644
--- a/sdk/lib/core/sequences.dart
+++ b/sdk/lib/core/sequences.dart
@@ -56,14 +56,7 @@ abstract class SequenceCollection<E> implements Collection<E>, Sequence<E> {
     return value;
   }
 
-  Collection<E> where(bool f(E element)) {
-    List<E> result = <E>[];
-    for (int i = 0; i < this.length; i++) {
-      E element = this[i];
-      if (f(element)) result.add(element);
-    }
-    return result;
-  }
+  Iterable<E> where(bool f(E element)) => new FilteredIterable<E>(this, f);
 
   bool every(bool f(E element)) {
     for (int i = 0; i < this.length; i++) {