Support comma operator in type guards

src/compiler/binder.ts

@@ -630,6 +630,8 @@ namespace ts {
                     return false;
                 case SyntaxKind.InstanceOfKeyword:
                     return isNarrowingExpression(expr.left);
+                case SyntaxKind.CommaToken:
+                    return isNarrowingExpression(expr.right);
             }
             return false;
         }

src/compiler/checker.ts

@@ -7677,6 +7677,8 @@ namespace ts {
                         break;
                     case SyntaxKind.InstanceOfKeyword:
                         return narrowTypeByInstanceof(type, expr, assumeTrue);
+                    case SyntaxKind.CommaToken:
+                        return narrowType(type, expr.right, assumeTrue);
                 }
                 return type;
             }