Add some support for the code-point code-unit distinction.

sdk/lib/io/string_stream.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.
 
 // Interface for decoders decoding binary data into string data. The
 // decoder keeps track of line breaks during decoding.
 abstract class _StringDecoder {
   // Add more binary data to be decoded. The ownership of the buffer
   // is transfered to the decoder and the caller most not modify it any more.
   int write(List<int> buffer);
@@ skipping 252 matching lines @@
     List<int> result = new Uint8List(size);
     _encodeString(string, result);
     return result;
   }
 
   static int _encodingSize(String string) => _encodeString(string, null);
 
   static int _encodeString(String string, List<int> buffer) {
     int pos = 0;
     int length = string.length;
+    // TODO(erikcorry): Use new iterator over charcodes.
     for (int i = 0; i < length; i++) {
       int additionalBytes;
       int charCode = string.charCodeAt(i);
       if (charCode <= 0x007F) {
         additionalBytes = 0;
         if (buffer != null) buffer[pos] = charCode;
       } else if (charCode <= 0x07FF) {
         // 110xxxxx (xxxxx is top 5 bits).
         if (buffer != null) buffer[pos] = ((charCode >> 6) & 0x1F) | 0xC0;
         additionalBytes = 1;
       } else if (charCode <= 0xFFFF) {
         // 1110xxxx (xxxx is top 4 bits)
         if (buffer != null) buffer[pos] = ((charCode >> 12) & 0x0F)| 0xE0;
         additionalBytes = 2;
       } else {
+        i++; // Skip surrogate pair.
         // 11110xxx (xxx is top 3 bits)
         if (buffer != null) buffer[pos] = ((charCode >> 18) & 0x07) | 0xF0;
         additionalBytes = 3;
       }
       pos++;
       if (buffer != null) {
         for (int i = additionalBytes; i > 0; i--) {
           // 10xxxxxx (xxxxxx is next 6 bits from the top).
           buffer[pos++] = ((charCode >> (6 * (i - 1))) & 0x3F) | 0x80;
         }
@@ skipping 235 matching lines @@
   bool _inputClosed = false;  // Is the underlying input stream closed?
   bool _closed = false;  // Is this stream closed.
   bool _eof = false;  // Has all data been read from the decoder?
   Timer _scheduledDataCallback;
   Timer _scheduledLineCallback;
   Timer _scheduledCloseCallback;
   Function _clientDataHandler;
   Function _clientLineHandler;
   Function _clientCloseHandler;
 }