Reapply change to use system code page on Windows to fix Pub.

sdk/lib/io/process.dart

Index: sdk/lib/io/process.dart
diff --git a/sdk/lib/io/process.dart b/sdk/lib/io/process.dart
index f2062df562aa9f937731c494d2eef21edfa52253..87ff798df7688a26411b6ced4aa0aca83c1a7ca2 100644
--- a/sdk/lib/io/process.dart
+++ b/sdk/lib/io/process.dart
@@ -148,7 +148,7 @@ class ProcessOptions {
    * This option is ignored for interactive processes started with
    * [:Process.start:].
    *
-   * The default stdoutEncoding is UTF_8.
+   * The default stdoutEncoding is SYSTEM.
    */
   Encoding stdoutEncoding;
 
@@ -159,7 +159,7 @@ class ProcessOptions {
    * This option is ignored for interactive processes started with
    * [:Process.start:].
    *
-   * The default stderrEncoding is UTF_8.
+   * The default stderrEncoding is SYSTEM.
    */
   Encoding stderrEncoding;