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

sdk/lib/io/string_stream.dart

Index: sdk/lib/io/string_stream.dart
diff --git a/sdk/lib/io/string_stream.dart b/sdk/lib/io/string_stream.dart
index c16815327dba39d7f330498dd09b29db38ee91b9..00ff3bce2cd611e59820a265d49d22c4e314d0d5 100644
--- a/sdk/lib/io/string_stream.dart
+++ b/sdk/lib/io/string_stream.dart
@@ -270,6 +270,7 @@ class _UTF8Encoder implements _StringEncoder {
   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);
@@ -285,6 +286,7 @@ class _UTF8Encoder implements _StringEncoder {
         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;