Tighten a couple type annotations in IterableMixin

sdk/lib/collection/set.dart

Index: sdk/lib/collection/set.dart
diff --git a/sdk/lib/collection/set.dart b/sdk/lib/collection/set.dart
index 74707611a7bbe6ca32cb97947dbf28bfe0ccb247..309355c15fef4249e3cbb6576edfade2a73b876e 100644
--- a/sdk/lib/collection/set.dart
+++ b/sdk/lib/collection/set.dart
@@ -232,7 +232,7 @@ abstract class SetMixin<E> implements Set<E> {
     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;
     }
@@ -240,7 +240,7 @@ abstract class SetMixin<E> implements Set<E> {
     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) {