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

tests/standalone/vmservice/test_helper.dart

Index: tests/standalone/vmservice/test_helper.dart
diff --git a/tests/standalone/vmservice/test_helper.dart b/tests/standalone/vmservice/test_helper.dart
index d199d05b8d539d1f091ed9e6f03e97f030b4dc16..f70e0f0dc7039d17eb3b5c951cd3f21ac1e0ae80 100644
--- a/tests/standalone/vmservice/test_helper.dart
+++ b/tests/standalone/vmservice/test_helper.dart
@@ -97,7 +97,7 @@ class TestLauncher {
       var portNumber;
       var blank;
       var first = true;
-      process.stdout.transform(new StringDecoder())
+      process.stdout.transform(UTF8.decoder)
                     .transform(new LineSplitter()).listen((line) {
         if (line.startsWith('VmService listening on port ')) {
           RegExp portExp = new RegExp(r"\d+");
@@ -116,7 +116,7 @@ class TestLauncher {
         }
         print(line);
       });
-      process.stderr.transform(new StringDecoder())
+      process.stderr.transform(UTF8.decoder)
                     .transform(new LineSplitter()).listen((line) {
         print(line);
       });