Typing fixes to eliminate casts/dcalls

tool/input_sdk/lib/collection/set.dart

Index: tool/input_sdk/lib/collection/set.dart
diff --git a/tool/input_sdk/lib/collection/set.dart b/tool/input_sdk/lib/collection/set.dart
index 4eb330606edef3c57519600671ce1ff33226d38b..717089e0b9e7357f8db4068c5b282c0414927c7e 100644
--- a/tool/input_sdk/lib/collection/set.dart
+++ b/tool/input_sdk/lib/collection/set.dart
@@ -125,7 +125,7 @@ abstract class SetMixin<E> implements Set<E> {
 
   E get single {
     if (length > 1) throw IterableElementError.tooMany();
-    Iterator it = iterator;
+    Iterator<E> it = iterator;
     if (!it.moveNext()) throw IterableElementError.noElement();
     E result = it.current;
     return result;
@@ -213,7 +213,7 @@ abstract class SetMixin<E> implements Set<E> {
   }
 
   E get first {
-    Iterator it = iterator;
+    Iterator<E> it = iterator;
     if (!it.moveNext()) {
       throw IterableElementError.noElement();
     }
@@ -221,7 +221,7 @@ abstract class SetMixin<E> implements Set<E> {
   }
 
   E get last {
-    Iterator it = iterator;
+    Iterator<E> it = iterator;
     if (!it.moveNext()) {
       throw IterableElementError.noElement();
     }