Optimize List.toList/.sublist and List.from on lists.

sdk/lib/core/list.dart

Index: sdk/lib/core/list.dart
diff --git a/sdk/lib/core/list.dart b/sdk/lib/core/list.dart
index 0b55b8ec0384ad67df417af79f14fc0b5257cab6..997b2c3f31ad09afcfa93a96a6975b1857ae5cfc 100644
--- a/sdk/lib/core/list.dart
+++ b/sdk/lib/core/list.dart
@@ -90,14 +90,7 @@ abstract class List<E> implements Iterable<E>, EfficientLength {
    * This constructor returns a growable list if [growable] is true;
    * otherwise, it returns a fixed-length list.
    */
-  factory List.from(Iterable other, { bool growable: true }) {
-    List<E> list = new List<E>();
-    for (E e in other) {
-      list.add(e);
-    }
-    if (growable) return list;
-    return makeListFixedLength(list);
-  }
+  external factory List.from(Iterable other, { bool growable: true });
 
   /**
    * Generates a list of values.