Let the inferrer know that lazy variables can also be null incase their initializer throws.

tests/language/throwing_lazy_variable_test.dart

Index: tests/language/throwing_lazy_variable_test.dart
===================================================================
--- tests/language/throwing_lazy_variable_test.dart	(revision 0)
+++ tests/language/throwing_lazy_variable_test.dart	(revision 0)
@@ -0,0 +1,22 @@
+// Copyright (c) 2013, 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.
+
+// Test that engines will not infer that [a] is always of type int.
+
+var a = foo();
+
+foo() {
+  () => 42;
+  if (true) throw 'Sorry';
+  return 42;
+}
+
+main() {
+  try {
+    a;
+  } catch (e) {
+    // Ignore.
+  }
+  if (a is num) throw 'Test failed';
+}