fix #28642, handling of top and bottom types in DDC

pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/types.dart

Index: pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/types.dart
diff --git a/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/types.dart b/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/types.dart
index abc476d54c70615a597a402851cb59b79606b444..7aa86153beba0f2f1f4d5082aea4d017bc07d680 100644
--- a/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/types.dart
+++ b/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/types.dart
@@ -655,16 +655,6 @@ isFunctionSubtype(ft1, ft2, isCovariant) => JS(
 
   // Check return type last, so that arity mismatched functions can be
   // definitively rejected.
-
-  // We allow any type to subtype a void return type, but not vice versa
-  if (ret2 === $_void) return true;
-  // Dart allows void functions to subtype dynamic functions, but not
-  // other functions.
-  // TODO(jmesserly): this check does not match our compile time subtype
-  // implementation. Reconcile.
-  if (ret1 === $_void) {
-    return ret2 === $dynamic || ret2 === $FutureOr;

[Leaf, 2017/02/06 21:11:59]

For now, can you keep this test?  Deleting it is what is causing the expectation
change in the runtime tests, and I'm concerned that this will break tests or
code.  This line allows us to return false when we test whether a function
returning void is a function returning int.  We actually do agree with the VM on
this, so it's safe to return false.  Otherwise we fall through to the next
check, where we return null, since we don't know whether were testing void
against int (safe, we agree with the VM) or Object against int (unsafe, we
disagree with the VM).

[Jennifer Messerly, 2017/02/06 22:50:12]

okay, sure, it should be _isTop(ret2) then?

[Leaf, 2017/02/06 22:55:54]

Yep, that would be the best fix.

-  }
   if (!$_isSubtype(ret1, ret2, $isCovariant)) return null;
   return true;
 })()''');
@@ -698,13 +688,14 @@ _subtypeMemo(f) => JS(
 final isSubtype = JS(
     '', '$_subtypeMemo((t1, t2) => (t1 === t2) || $_isSubtype(t1, t2, true))');
 
-_isBottom(type) => JS('bool', '# == #', type, bottom);
+_isBottom(type) => JS('bool', '# == # || # == #', type, bottom, type, Null);
 
 _isTop(type) {
   if (JS('bool', '# === #', getGenericClass(type), getGenericClass(FutureOr))) {
     return _isTop(JS('', '#[0]', getGenericArgs(type)));
   }
-  return JS('bool', '# == # || # == #', type, Object, type, dynamic);
+  return JS('bool', '# == # || # == # || # == #',
+      type, Object, type, dynamic, type, _void);
 }
 
 _isSubtype(t1, t2, isCovariant) => JS(