Reland "VM: Handle null-comparisons in the flow graph type propagation"

runtime/lib/integers.dart

Index: runtime/lib/integers.dart
diff --git a/runtime/lib/integers.dart b/runtime/lib/integers.dart
index ff755e9c8dd2f9e65f9ef477f8a71029a62fff34..9af198912a223f71735e3c1418da8a35e79e236d 100644
--- a/runtime/lib/integers.dart
+++ b/runtime/lib/integers.dart
@@ -2,6 +2,10 @@
 // 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 marker interface represents 64-bit integers in the compiler for type
+// propagation and range analysis.  It is implemented by _Smi and _Mint.
+abstract class _int64 implements int {}
+
 abstract class _IntegerImplementation {
   // The Dart class _Bigint extending _IntegerImplementation requires a
   // default constructor.
@@ -454,7 +458,7 @@ abstract class _IntegerImplementation {
   }
 }
 
-class _Smi extends _IntegerImplementation implements int {
+class _Smi extends _IntegerImplementation implements int, _int64 {
   factory _Smi._uninstantiable() {
     throw new UnsupportedError("_Smi can only be allocated by the VM");
   }
@@ -654,7 +658,7 @@ class _Smi extends _IntegerImplementation implements int {
 }
 
 // Represents integers that cannot be represented by Smi but fit into 64bits.
-class _Mint extends _IntegerImplementation implements int {
+class _Mint extends _IntegerImplementation implements int, _int64 {
   factory _Mint._uninstantiable() {
     throw new UnsupportedError("_Mint can only be allocated by the VM");
   }