Split dart:builtin into dart:builtin and dart:io.

tests/standalone/src/StringStreamTest.dart

 // Copyright (c) 2011, 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,
                     0xdf, 0xbf,
                     0xe0, 0xa0, 0x80,
                     0xef, 0xbf, 0xbf];
   InputStream s = new ListInputStream(data);
   StringInputStream stream = new StringInputStream(s);
   void stringData() {
@@ skipping 132 matching lines @@
   s.write("Line1\nLine2\r\nLine3\rLi".charCodes());
 }
 
 main() {
   testUtf8();
   testLatin1();
   testAscii();
   testReadLine1();
   testReadLine2();
 }