Check for null type instead of null keyword

Author: ahejlsberg

src/compiler/checker.ts

@@ -13690,7 +13690,7 @@ namespace ts {
                     const doubleEquals = operator === SyntaxKind.EqualsEqualsToken || operator === SyntaxKind.ExclamationEqualsToken;
                     const facts = doubleEquals ?
                         assumeTrue ? TypeFacts.EQUndefinedOrNull : TypeFacts.NEUndefinedOrNull :
-                        value.kind === SyntaxKind.NullKeyword ?
+                        valueType.flags & TypeFlags.Null ?
                             assumeTrue ? TypeFacts.EQNull : TypeFacts.NENull :
                             assumeTrue ? TypeFacts.EQUndefined : TypeFacts.NEUndefined;
                     return getTypeWithFacts(type, facts);