Make dart:collection strong-mode clean.

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 4dd32cbf1368d19a7264c9decc85c52370312fd1..499225dca2b1f9fa40493e9091ca2571e671c4e1 100644
--- a/sdk/lib/collection/set.dart
+++ b/sdk/lib/collection/set.dart
@@ -33,7 +33,7 @@ abstract class SetMixin<E> implements Set<E> {
 
   bool contains(Object element);
 
-  E lookup(E element);
+  E lookup(Object element);
 
   bool remove(Object element);
 
@@ -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();
     }