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

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

 // Copyright (c) 2015, 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.
 
 /// This library defines the representation of runtime types.
 part of dart._runtime;
 
 final metadata = JS('', 'Symbol("metadata")');
 
 /// The symbol used to store the cached `Type` object associated with a class.
@@ skipping 637 matching lines @@
     if (n1 === void 0) {
       return ($isCovariant) ? false : null;
     }
     if (!$_isSubtype(n2, n1, !$isCovariant)) {
       return null;
     }
   }
 
   // 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;
 })()''');
 
 /// TODO(leafp): This duplicates code in operations.dart.
 /// I haven't found a way to factor it out that makes the
 /// code generator happy though.
 _subtypeMemo(f) => JS(
     '',
     '''(() => {
@@ skipping 13 matching lines @@
   };
 })()''');
 
 /// Returns true if [t1] <: [t2].
 /// Returns false if [t1] </: [t2] in both spec and strong mode
 /// Returns undefined if [t1] </: [t2] in strong mode, but spec
 ///  mode may differ
 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(
     '',
     '''(() => {
   if ($t1 === $t2) return true;
 
   // Trivially true.
   if ($_isTop($t2) || $_isBottom($t1)) {
     return true;
@@ skipping 152 matching lines @@
     return true;
   }
 
   let typeArgs = $getGenericArgs($type);
   if (!typeArgs) return true;
   for (let t of typeArgs) {
     if (t != $Object && t != $dynamic) return false;
   }
   return true;
 })()''');