Make dart:collection strong-mode clean.

sdk/lib/collection/iterable.dart

Index: sdk/lib/collection/iterable.dart
diff --git a/sdk/lib/collection/iterable.dart b/sdk/lib/collection/iterable.dart
index c0d2ee053be9ec890ab9bff660b35a4085176594..26ccb5ca3b9d776535c06574c57589a8e8b18d72 100644
--- a/sdk/lib/collection/iterable.dart
+++ b/sdk/lib/collection/iterable.dart
@@ -121,7 +121,7 @@ abstract class IterableMixin<E> implements Iterable<E> {
   }
 
   E get first {
-    Iterator it = iterator;
+    Iterator<E> it = iterator;
     if (!it.moveNext()) {
       throw IterableElementError.noElement();
     }
@@ -129,7 +129,7 @@ abstract class IterableMixin<E> implements Iterable<E> {
   }
 
   E get last {
-    Iterator it = iterator;
+    Iterator<E> it = iterator;
     if (!it.moveNext()) {
       throw IterableElementError.noElement();
     }
@@ -141,7 +141,7 @@ abstract class IterableMixin<E> implements Iterable<E> {
   }
 
   E get single {
-    Iterator it = iterator;
+    Iterator<E> it = iterator;
     if (!it.moveNext()) throw IterableElementError.noElement();
     E result = it.current;
     if (it.moveNext()) throw IterableElementError.tooMany();