Make int and double nullable by default

test/dart_codegen/expect/collection/set.dart

 part of dart.collection;
  abstract class SetMixin<E> implements Set<E> {bool add(E element);
  bool contains(Object element);
  E lookup(E element);
  bool remove(Object element);
  Iterator<E> get iterator;
  Set<E> toSet();
  int get length;
  bool get isEmpty => length == 0;
  bool get isNotEmpty => length != 0;
@@ skipping 161 matching lines @@
 ) {
   for (E element in this) {
     if (test(element)) return element;
     }
    if (orElse != null) return orElse();
    throw IterableElementError.noElement();
   }
  E lastWhere(bool test(E value), {
   E orElse()}
 ) {
-  E result = ((__x49) => DDC$RT.cast(__x49, Null, E, "CastLiteral", """line 244, column 16 of dart:collection/set.dart: """, __x49 is E, false))(null);
+  E result = null;
    bool foundMatching = false;
    for (E element in this) {
     if (test(element)) {
       result = element;
        foundMatching = true;
       }
     }
    if (foundMatching) return result;
    if (orElse != null) return orElse();
    throw IterableElementError.noElement();
   }
  E singleWhere(bool test(E value)) {
-  E result = ((__x50) => DDC$RT.cast(__x50, Null, E, "CastLiteral", """line 258, column 16 of dart:collection/set.dart: """, __x50 is E, false))(null);
+  E result = null;
    bool foundMatching = false;
    for (E element in this) {
     if (test(element)) {
       if (foundMatching) {
         throw IterableElementError.tooMany();
         }
        result = element;
        foundMatching = true;
       }
     }
    if (foundMatching) return result;
    throw IterableElementError.noElement();
   }
  E elementAt(int index) {
   if (index is! int) throw new ArgumentError.notNull("index");
    RangeError.checkNotNegative(index, "index");
    int elementIndex = 0;
    for (E element in this) {
     if (index == elementIndex) return element;
      elementIndex++;
     }
    throw new RangeError.index(index, this, "index", null, elementIndex);
   }
 }
  abstract class SetBase<E> extends SetMixin<E> {static String setToString(Set set) => IterableBase.iterableToFullString(set, '{', '}');
 }