[dart:io] Move Platform.ansiSupported to {Stdin,Stdout}.supportsAnsiEscapes

tests/standalone/io/ansi_supported_test.dart

 // Copyright (c) 2017, 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.
 
 import 'dart:io';
 
 import "package:expect/expect.dart";
 
-main() {
+testStdout(Stdout s) {
   try {
-    Platform.ansiSupported;
-  } catch (e, s) {
-    Expect.fail("Platform.ansiSupported threw: $e\n$s\n");
+    s.ansiSupported;
+  } catch (e, st) {
+    Expect.fail("$s.ansiSupported threw: $e\n$st\n");
   }
-  Expect.isNotNull(Platform.ansiSupported);
-  Expect.isTrue(Platform.ansiSupported is bool);
-  if (stdout.hasTerminal && Platform.ansiSupported) {
-    stdout.writeln('\x1b[31mThis text has a red foreground using SGR.31.');
-    stdout.writeln('\x1b[39mThis text has restored the foreground color.');
+  Expect.isNotNull(s.ansiSupported);
+  Expect.isTrue(s.ansiSupported is bool);
+  if (s.ansiSupported) {
+    s.writeln('\x1b[31mThis text has a red foreground using SGR.31.');
+    s.writeln('\x1b[39mThis text has restored the foreground color.');
   } else {
-    stdout.writeln('ANSI codes not supported on this platform');
+    s.writeln('ANSI codes not supported on this platform');
   }
 }
+
+main() {
+  testStdout(stdout);
+  testStdout(stderr);
+  try {
+    stdin.ansiSupported;
+  } catch (e, st) {
+    Expect.fail("stdin.ansiSupported threw: $e\n$st\n");
+  }
+  Expect.isNotNull(stdin.ansiSupported);
+  Expect.isTrue(stdin.ansiSupported is bool);
+}