Add method available to StringInputStream

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 89 matching lines @@
 }
 
 void testReadLine2() {
   InputStream s = new ListInputStream();
   StringInputStream stream = new StringInputStream(s);
   var stage = 0;
 
   void stringData() {
     var line;
     if (stage == 0) {
+      Expect.equals(21, stream.available());
       line = stream.readLine();
       Expect.equals("Line1", line);
+      Expect.equals(15, stream.available());
       line = stream.readLine();
       Expect.equals("Line2", line);
+      Expect.equals(8, stream.available());
       line = stream.readLine();
       Expect.equals("Line3", line);
       line = stream.readLine();
+      Expect.equals(2, stream.available());
       Expect.equals(null, line);
       stage++;
       s.write("ne4\n".charCodes());
     } else if (stage == 1) {
+      Expect.equals(6, stream.available());
       line = stream.readLine();
       Expect.equals("Line4", line);
+      Expect.equals(0, stream.available());
       line = stream.readLine();
       Expect.equals(null, line);
       stage++;
       s.write("\n\n\r\n\r\n\r\r".charCodes());
     } else if (stage == 2) {
       // Expect 5 empty lines. As long as the stream is not closed the
       // final \r cannot be interpreted as a end of line.
+      Expect.equals(8, stream.available());
       for (int i = 0; i < 5; i++) {
         line = stream.readLine();
         Expect.equals("", line);
       }
+      Expect.equals(1, stream.available());
       line = stream.readLine();
       Expect.equals(null, line);
       stage++;
       s.markEndOfStream();
     } else if (stage == 3) {
       // The final \r can now be interpreted as an end of line.
+      Expect.equals(1, stream.available());
       line = stream.readLine();
       Expect.equals("", line);
       line = stream.readLine();
       Expect.equals(null, line);
       stage++;
     }
   }
 
   void streamClosed() {
     Expect.equals(4, stage);
     Expect.equals(true, stream.closed);
   }
 
   stream.onLine = stringData;
   stream.onClosed = streamClosed;
   s.write("Line1\nLine2\r\nLine3\rLi".charCodes());
 }
 
 main() {
   testUtf8();
   testLatin1();
   testAscii();
   testReadLine1();
   testReadLine2();
 }