Tighten a couple type annotations in IterableMixin

sdk/lib/collection/set.dart

 // Copyright (c) 2014, 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.
 
 /**
  * Base implementations of [Set].
  */
 part of dart.collection;
 
 /**
@@ skipping 214 matching lines @@
     if (!it.moveNext()) {
       throw IterableElementError.noElement();
     }
     E result;
     do {
       result = it.current;
     } while(it.moveNext());
     return result;
   }
 
-  dynamic firstWhere(bool test(E value), { Object orElse() }) {
+  E firstWhere(bool test(E value), { E orElse() }) {
     for (E element in this) {
       if (test(element)) return element;
     }
     if (orElse != null) return orElse();
     throw IterableElementError.noElement();
   }
 
-  dynamic lastWhere(bool test(E value), { Object orElse() }) {
+  E lastWhere(bool test(E value), { E orElse() }) {
     E result = null;
     bool foundMatching = false;
     for (E element in this) {
       if (test(element)) {
         result = element;
         foundMatching = true;
       }
     }
     if (foundMatching) return result;
     if (orElse != null) return orElse();
@@ skipping 48 matching lines @@
 abstract class SetBase<E> extends SetMixin<E> {
   /**
    * Convert a `Set` to a string as `{each, element, as, string}`.
    *
    * Handles circular references where converting one of the elements
    * to a string ends up converting [set] to a string again.
    */
   static String setToString(Set set) =>
       IterableBase.iterableToFullString(set, '{', '}');
 }