Add a strong mode implementation of LUB to fasta; use it to infer ?:

pkg/kernel/lib/type_environment.dart

Index: pkg/kernel/lib/type_environment.dart
diff --git a/pkg/kernel/lib/type_environment.dart b/pkg/kernel/lib/type_environment.dart
index 8db70bb9374718e1cfe62a83712dddcc7f1021d5..edefd06aee3a2058eb41122d63b5497ce07dbdec 100644
--- a/pkg/kernel/lib/type_environment.dart
+++ b/pkg/kernel/lib/type_environment.dart
@@ -143,17 +143,22 @@ abstract class SubtypeTester {
   InterfaceType get rawFunctionType;
   ClassHierarchy get hierarchy;
 
+  /// Determines if the given type is at the bottom of the type hierarchy.  May
+  /// be overridden in subclasses.
+  bool isBottom(DartType type) => type is BottomType;

[ahe, 2017/05/02 18:32:57]

This is Null, right?

[Paul Berry, 2017/05/03 17:31:25]

I discussed this with Leaf yesterday.  The intention is that Null and Bottom
should be represented as distinct types, with bottom as a subtype of Null.

Eventually, once we have non-nullability, Null will be a subtype of all nullable
types but not of non-nullable types, so it won't be true bottom.

For now I'm just trying to refactor the existing code without changing
functionality.

+
+  /// Determines if the given type is at the top of the type hierarchy.  May be
+  /// overridden in subclasses.
+  bool isTop(DartType type) =>
+      type is DynamicType || type is VoidType || type == objectType;
+
   /// Returns true if [subtype] is a subtype of [supertype].
   bool isSubtypeOf(DartType subtype, DartType supertype) {
     subtype = subtype.unalias;
     supertype = supertype.unalias;
     if (identical(subtype, supertype)) return true;
-    if (subtype is BottomType) return true;
-    if (supertype is DynamicType ||
-        supertype is VoidType ||
-        supertype == objectType) {
-      return true;
-    }
+    if (isBottom(subtype)) return true;
+    if (isTop(supertype)) return true;
     if (subtype is InterfaceType && supertype is InterfaceType) {
       var upcastType =
           hierarchy.getTypeAsInstanceOf(subtype, supertype.classNode);