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

tests/standalone/coverage_test.dart

Index: tests/standalone/coverage_test.dart
diff --git a/tests/standalone/coverage_test.dart b/tests/standalone/coverage_test.dart
index ecf92eaf7ea3de55bfcc69e9ddfa8237b4303c2c..c9a5c5fc152850d932b31ab3ee729e347226fe76 100644
--- a/tests/standalone/coverage_test.dart
+++ b/tests/standalone/coverage_test.dart
@@ -65,11 +65,11 @@ void main() {
     coverageToolProcess = process;
     coverageToolProcess.stdin.close();
     var stdoutStringStream = coverageToolProcess.stdout
-        .transform(new StringDecoder())
+        .transform(UTF8.decoder)
         .transform(new LineSplitter());
 
     var stderrStringStream = coverageToolProcess.stderr
-        .transform(new StringDecoder())
+        .transform(UTF8.decoder)
         .transform(new LineSplitter());
 
     // Wait for 3 future events: stdout and stderr streams of the coverage