Use AssertionError rather than Exceptions for unexpected behaviour in io.

Follow up to https://dart-review.googlesource.com/c/sdk/+/257140/comments/38b33a34_5c1914ff.

Unreachable code in error handling.

Change-Id: I9e8a5b19372d63a604efcdf9856a618e5f7b520f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/258513
Commit-Queue: Kallen Tu <kallentu@google.com>
Reviewed-by: Lasse Nielsen <lrn@google.com>

sdk/lib/_internal/vm/bin/socket_patch.dart

@@ -1549,23 +1549,23 @@ class _NativeSocket extends _NativeSocketNativeWrapper with _ServiceObject {
   static createError(error, String message,
       [InternetAddress? address, int? port]) {
     if (error is OSError) {
-      return new SocketException(message,
+      return SocketException(message,
           osError: error, address: address, port: port);
     } else if (error is List) {
       assert(isErrorResponse(error));
       switch (error[0]) {
         case _illegalArgumentResponse:
-          return new ArgumentError();
+          return ArgumentError();
         case _osErrorResponse:
-          return new SocketException(message,
-              osError: new OSError(error[2], error[1]),
+          return SocketException(message,
+              osError: OSError(error[2], error[1]),
               address: address,
               port: port);
         default:
-          return new Exception("Unknown error");
+          return AssertionError("Unknown error");
       }
     } else {
-      return new SocketException(message, address: address, port: port);
+      return SocketException(message, address: address, port: port);
     }
   }
 

sdk/lib/io/common.dart

@@ -28,7 +28,7 @@ void _checkForErrorResponse(Object? response, String message, String path) {
       case _fileClosedResponse:
         throw FileSystemException("File closed", path);
       default:
-        throw Exception("Unknown error");
+        throw AssertionError("Unknown error");
     }
   }
 }

sdk/lib/io/directory_impl.dart

@@ -255,7 +255,7 @@ class _Directory extends FileSystemEntity implements Directory {
               response[_osErrorResponseErrorCode] as int);
           throw FileSystemException(message, path, err);
         default:
-          throw Exception("Unknown error");
+          throw AssertionError("Unknown error");
       }
     }
   }

sdk/lib/io/link.dart

@@ -266,7 +266,7 @@ class _Link extends FileSystemEntity implements Link {
               response[_osErrorResponseErrorCode] as int);
           throw FileSystemException(message, path, err);
         default:
-          throw Exception("Unknown error");
+          throw AssertionError("Unknown error");
       }
     }
   }