Don't crash when decoding astral plane UTF-8.

tests/standalone/io/string_stream_test.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");
 #import("dart:isolate");
 
 void testUtf8() {
   List<int> data = [0x01,
                     0x7f,
                     0xc2, 0x80,
                     0xdf, 0xbf,
                     0xe0, 0xa0, 0x80,
-                    0xef, 0xbf, 0xbf];
+                    0xef, 0xbf, 0xbf,
+                    0xf0, 0x9d, 0x84, 0x9e];
   ListInputStream s = new ListInputStream();
   s.write(data);
   s.markEndOfStream();
   StringInputStream stream = new StringInputStream(s);
   void stringData() {
     String s = stream.read();
-    Expect.equals(6, s.length);
+    Expect.equals(8, s.length);
     Expect.equals(new String.fromCharCodes([0x01]), s[0]);
     Expect.equals(new String.fromCharCodes([0x7f]), s[1]);
     Expect.equals(new String.fromCharCodes([0x80]), s[2]);
     Expect.equals(new String.fromCharCodes([0x7ff]), s[3]);
     Expect.equals(new String.fromCharCodes([0x800]), s[4]);
     Expect.equals(new String.fromCharCodes([0xffff]), s[5]);
+    Expect.equals(new String.fromCharCodes([0xffff]), s[5]);
+
+    // Surrogate pair for U+1D11E.
+    Expect.equals(new String.fromCharCodes([0xd834]), s[6]);
+    Expect.equals(new String.fromCharCodes([0xdd1e]), s[7]);
   }
   stream.onData = stringData;
 }
 
 void testLatin1() {
   List<int> data = [0x01,
                     0x7f,
                     0x44, 0x61, 0x72, 0x74,
                     0x80,
                     0xff];
@@ skipping 255 matching lines @@
   var errors = 0;
   var stream = new StringInputStream(new ErrorInputStream());
   stream.onError = (e) {
     errors++;
     Expect.isTrue(e is TestException);
   };
   Expect.equals(1, errors);
 }
 
 testEncodingErrorWithHandler() {
-  var port = new ReceivePort();    
+  var port = new ReceivePort();
   var errors = 0;
   var expected = [206, 187, 120, 46, 32, 120, 10];
   ListInputStream input = new ListInputStream();
   input.write(expected);
   var stringStream = new StringInputStream(input, Encoding.ASCII);
   stringStream.onData = () {
     Expect.fail("We should not get any data");
   };
   stringStream.onError = (e) {
     port.close();
     Expect.isTrue(e is Exception);
   };
 }
 
 
 main() {
   testUtf8();
   testLatin1();
   testAscii();
   testReadLine1();
   testReadLine2();
   testReadChunks();
   testReadMixed();
   testErrorHandler();
   testEncodingErrorWithHandler();
 }