Fix handling of null/bottom in front_end type inference.

pkg/front_end/testcases/inference/list_literals_can_infer_null_bottom.dart.strong.expect

Index: pkg/front_end/testcases/inference/list_literals_can_infer_null_bottom.dart.strong.expect
diff --git a/pkg/front_end/testcases/inference/list_literals_can_infer_null_bottom.dart.strong.expect b/pkg/front_end/testcases/inference/list_literals_can_infer_null_bottom.dart.strong.expect
new file mode 100644
index 0000000000000000000000000000000000000000..463943cc589665a8c957180b775f23405b4f9714
--- /dev/null
+++ b/pkg/front_end/testcases/inference/list_literals_can_infer_null_bottom.dart.strong.expect
@@ -0,0 +1,11 @@
+library test;
+import self as self;
+import "dart:core" as core;
+
+static method test1() → dynamic {
+  core::List<core::Null> x = <core::Null>[null];
+  x.add(42);
+}
+static method main() → dynamic {
+  self::test1();
+}