Infer Null for return type of functions with empty returns.

tests/lib_strong/collection/hash_set_test.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import "dart:collection";
 import "package:expect/expect.dart";
 
 void main() {
   // Test customized sets.
   // Regression test for issue http://dartbug.com/18109
 
-  hash(s) => s.toLowerCase().hashCode;
-  equals(a, b) => a.toLowerCase() == b.toLowerCase();
+  int hash(s) => s.toLowerCase().hashCode;
+  bool equals(a, b) => a.toLowerCase() == b.toLowerCase();
 
   for (var m in [
     new HashSet<String>(equals: equals, hashCode: hash),
     new LinkedHashSet<String>(equals: equals, hashCode: hash),
   ]) {
     m.add("Abel");
     var prev = "Abel";
     for (var key in ["Abel", "abel", "ABEL", "Abel"]) {
       Expect.isTrue(m.contains(key), "contains $key in ${m.runtimeType} $m");
       Expect.equals(prev, m.lookup(key), "lookup $key in ${m.runtimeType} $m");
       Expect.isTrue(m.remove(key), "remove $key in ${m.runtimeType} $m");
       m.add(key);
       prev = key;
     }
   }
 
-  abshash(n) => n.abs();
-  abseq(a, b) => a.abs() == b.abs();
+  int abshash(n) => n.abs();
+  bool abseq(a, b) => a.abs() == b.abs();
   for (var m in [
     new HashSet<int>(equals: abseq, hashCode: abshash),
     new LinkedHashSet<int>(equals: abseq, hashCode: abshash),
   ]) {
     m.add(1);
     var prev = 1;
     for (var key in [1, -1, 1]) {
       Expect.isTrue(m.contains(key), "contains $key in ${m.runtimeType} $m");
       Expect.equals(prev, m.lookup(key), "lookup $key in ${m.runtimeType} $m");
       Expect.isTrue(m.remove(key), "remove $key in ${m.runtimeType} $m");
       m.add(key);
       prev = key;
     }
   }
 }