By default use current code page on Windows for decoding string

runtime/bin/process_patch.dart

 // Copyright (c) 2012, 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.
 
+patch class _WindowsCodePageDecoder {
+  /* patch */ static String _decodeBytes(List<int> bytes)
+      native "SystemEncodingToString";
+}
+
+
+patch class _WindowsCodePageEncoder {
+  /* patch */ static List<int> _encodeString(String string)
+      native "StringToSystemEncoding";
+}
+
+
 patch class Process {
   /* patch */ static Future<Process> start(String executable,
                                            List<String> arguments,
                                            [ProcessOptions options]) {
     _ProcessImpl process = new _ProcessImpl(executable, arguments, options);
     return process._start();
   }
 
   /* patch */ static Future<ProcessResult> run(String executable,
                                                List<String> arguments,
@@ skipping 256 matching lines @@
 // _NonInteractiveProcess is a wrapper around an interactive process
 // that buffers output so it can be delivered when the process exits.
 // _NonInteractiveProcess is used to implement the Process.run
 // method.
 class _NonInteractiveProcess {
   _NonInteractiveProcess(String path,
                          List<String> arguments,
                          ProcessOptions options) {
     _completer = new Completer<ProcessResult>();
     // Extract output encoding options and verify arguments.
-    var stdoutEncoding = Encoding.UTF_8;
-    var stderrEncoding = Encoding.UTF_8;
+    var stdoutEncoding = Encoding.SYSTEM;
+    var stderrEncoding = Encoding.SYSTEM;
     if (options != null) {
       if (options.stdoutEncoding != null) {
         stdoutEncoding = options.stdoutEncoding;
         if (stdoutEncoding is !Encoding) {
           throw new ArgumentError(
               'stdoutEncoding option is not an encoding: $stdoutEncoding');
         }
       }
       if (options.stderrEncoding != null) {
         stderrEncoding = options.stderrEncoding;
@@ skipping 69 matching lines @@
 
 class _ProcessResult implements ProcessResult {
   const _ProcessResult(int this.exitCode,
                        String this.stdout,
                        String this.stderr);
 
   final int exitCode;
   final String stdout;
   final String stderr;
 }