Remove Encoding-enum from dart:io and add interface in dart:convert.

sdk/lib/_internal/lib/io_patch.dart

Index: sdk/lib/_internal/lib/io_patch.dart
diff --git a/sdk/lib/_internal/lib/io_patch.dart b/sdk/lib/_internal/lib/io_patch.dart
index 80725b894e137570e04a7ebcf106b08232a90b98..0bd68ac8d0d0e9c4e4aa3008636da5c4e120ff0d 100644
--- a/sdk/lib/_internal/lib/io_patch.dart
+++ b/sdk/lib/_internal/lib/io_patch.dart
@@ -208,8 +208,8 @@ patch class Process {
        Map<String, String> environment,
        bool includeParentEnvironment: true,
        bool runInShell: false,
-       Encoding stdoutEncoding: Encoding.SYSTEM,
-       Encoding stderrEncoding: Encoding.SYSTEM}) {
+       Encoding stdoutEncoding: SYSTEM_ENCODING,
+       Encoding stderrEncoding: SYSTEM_ENCODING}) {
     throw new UnsupportedError("Process.run");
   }
 
@@ -220,8 +220,8 @@ patch class Process {
        Map<String, String> environment,
        bool includeParentEnvironment: true,
        bool runInShell: false,
-       Encoding stdoutEncoding: Encoding.SYSTEM,
-       Encoding stderrEncoding: Encoding.SYSTEM}) {
+       Encoding stdoutEncoding: SYSTEM_ENCODING,
+       Encoding stderrEncoding: SYSTEM_ENCODING}) {
     throw new UnsupportedError("Process.runSync");
   }
 }