Add writeString method to OutputStream

tests/standalone/src/io/StringStreamTest.dart

 // Copyright (c) 2012, 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");
 
 void testUtf8() {
   List<int> data = [0x01,
                     0x7f,
                     0xc2, 0x80,
@@ skipping 19 matching lines @@
 
 void testLatin1() {
   List<int> data = [0x01,
                     0x7f,
                     0x44, 0x61, 0x72, 0x74,
                     0x80,
                     0xff];
   InputStream s = new ListInputStream();
   s.write(data);
   s.markEndOfStream();
-  StringInputStream stream = new StringInputStream(s, "ISO-8859-1");
+  StringInputStream stream = new StringInputStream(s, Encoding.ISO_8859_1);
   void stringData() {
     String s = stream.read();
     Expect.equals(8, s.length);
     Expect.equals(new String.fromCharCodes([0x01]), s[0]);
     Expect.equals(new String.fromCharCodes([0x7f]), s[1]);
     Expect.equals("Dart", s.substring(2, 6));
     Expect.equals(new String.fromCharCodes([0x80]), s[6]);
     Expect.equals(new String.fromCharCodes([0xff]), s[7]);
   }
   stream.onData = stringData;
 }
 
 void testAscii() {
   List<int> data = [0x01,
                     0x44, 0x61, 0x72, 0x74,
                     0x7f];
   InputStream s = new ListInputStream();
   s.write(data);
   s.markEndOfStream();
-  StringInputStream stream = new StringInputStream(s, "ASCII");
+  StringInputStream stream =
+      new StringInputStream(s, Encoding.ASCII);
   void stringData() {
     String s = stream.read();
     Expect.equals(6, s.length);
     Expect.equals(new String.fromCharCodes([0x01]), s[0]);
     Expect.equals("Dart", s.substring(1, 5));
     Expect.equals(new String.fromCharCodes([0x7f]), s[5]);
   }
   stream.onData = stringData;
 }
 
@@ skipping 93 matching lines @@
   s.write("Line1\nLine2\r\nLine3\rLi".charCodes());
 }
 
 main() {
   testUtf8();
   testLatin1();
   testAscii();
   testReadLine1();
   testReadLine2();
 }