Change VM's string-buffer patch to use a Uin16Array as backing buffer.

sdk/lib/core/string_buffer.dart

Index: sdk/lib/core/string_buffer.dart
diff --git a/sdk/lib/core/string_buffer.dart b/sdk/lib/core/string_buffer.dart
index 07d3277066e8832c6077c805a8ed37aee8ebe77d..47fc8db65d58b4eb88421760b3120eed32053cb9 100644
--- a/sdk/lib/core/string_buffer.dart
+++ b/sdk/lib/core/string_buffer.dart
@@ -31,8 +31,12 @@ class StringBuffer implements StringSink {
   @deprecated
   void add(Object obj) => write(obj);
 
+  /// Adds the contents of [obj], converted to a string, to the buffer.
   external void write(Object obj);
 
+  /// Adds the string representation of [charCode] to the buffer.
+  external void writeCharCode(int charCode);
+
   void writeAll(Iterable objects) {
     for (Object obj in objects) write(obj);
   }
@@ -52,11 +56,6 @@ class StringBuffer implements StringSink {
     writeCharCode(charCode);
   }
 
-  /// Adds the string representation of [charCode] to the buffer.
-  void writeCharCode(int charCode) {
-    write(new String.fromCharCode(charCode));
-  }
-
   /**
    * Adds all items in [objects] to the buffer.
    *