Fix handling of null/bottom in front_end type inference.

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

Index: pkg/front_end/testcases/inference/list_literals.dart.strong.expect
diff --git a/pkg/front_end/testcases/inference/list_literals.dart.strong.expect b/pkg/front_end/testcases/inference/list_literals.dart.strong.expect
new file mode 100644
index 0000000000000000000000000000000000000000..17ab75affeb6b2c31381e6ae77b026fe3459022b
--- /dev/null
+++ b/pkg/front_end/testcases/inference/list_literals.dart.strong.expect
@@ -0,0 +1,21 @@
+library test;
+import self as self;
+import "dart:core" as core;
+
+static method test1() → dynamic {
+  core::List<core::int> x = <core::int>[1, 2, 3];
+  x.add("hi");
+  x.add(4.0);
+  x.add(4);
+  core::List<core::num> y = x;
+}
+static method test2() → dynamic {
+  core::List<core::num> x = <core::num>[1, 2.0, 3];
+  x.add("hi");
+  x.add(4.0);
+  core::List<core::int> y = x;
+}
+static method main() → dynamic {
+  self::test1();
+  self::test2();
+}