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

runtime/lib/string_buffer_patch.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.
 
 patch class StringBuffer {
-  List<String> _buffer;
+  List<int> _buffer;
   int _length;
 
   /// Creates the string buffer with an initial content.
   /* patch */ StringBuffer([Object content = ""]) {
-    _buffer = new List<String>();
+    _buffer = _newBuffer(16);
     _length = 0;
     write(content);
   }
 
   /* patch */ int get length => _length;
 
   /// Adds [obj] to the buffer.
   /* patch */ void write(Object obj) {
-    // TODO(srdjan): The following four lines could be replaced by
-    // '$obj', but apparently this is too slow on the Dart VM.
     String str;
     if (obj is String) {
       str = obj;
     } else {
+      // TODO(srdjan): The following four lines could be replaced by
+      // '$obj', but apparently this is too slow on the Dart VM.
       str = obj.toString();
       if (str is! String) {
         throw new ArgumentError('toString() did not return a string');
       }
     }
     if (str.isEmpty) return;
-    _buffer.add(str);
+    _ensureCapacity(str.length);
+    for (int i = 0; i < str.length; i++) {
+      _buffer[_length + i] = str.codeUnitAt(i);
+    }
     _length += str.length;
   }
 
-  /// Clears the string buffer.
+  /* patch */ writeCharCode(int charCode) {
+    if (charCode < 0 || charCode > 0x10FFFF) {
+      throw new RangeError.range(charCode, 0, 0x10FFFF);
+    }
+    if (charCode <= 0xFFFF) {
+      _ensureCapacity(1);
+      _buffer[_length++] = charCode;
+    } else {
+      _ensureCapacity(2);
+      int bits = charCode - 0x10000;
+      _buffer[_length++] = 0xD800 | (bits >> 10);
+      _buffer[_length++] = 0xDC00 | (bits & 0x3FF);
+    }
+  }
+
+  /** Makes the buffer empty. */
   /* patch */ void clear() {
-    _buffer = new List<String>();
     _length = 0;
   }
 
-  /// Returns the contents of buffer as a concatenated string.
+  /** Returns the contents of buffer as a string. */
   /* patch */ String toString() {
-    if (_buffer.length == 0) return "";
-    if (_buffer.length == 1) return _buffer[0];
-    String result = _StringBase.concatAll(_buffer);
-    _buffer.clear();
-    _buffer.add(result);
-    // Since we track the length at each add operation, there is no
-    // need to update it in this function.
-    return result;
+    if (_length == 0) return "";
+    return _create(_buffer, _length);
   }
+
+  /** Ensures that the buffer has enough capacity to contain n code units. */
+  void _ensureCapacity(int n) {
+    int requiredCapacity = _length + n;
+    if (requiredCapacity <= _buffer.length) return;
+    int newCapacity = _buffer.length;
+    do {
+      newCapacity *= 2;
+    } while (newCapacity < requiredCapacity);
+    List<int> newBuffer = _newBuffer(newCapacity);
+    newBuffer.setRange(0, _length, _buffer);
+    _buffer = newBuffer;
+  }
+
+  /**
+   * Create a [String] from the UFT-16 code units in buffer.
+   */
+  static String _create(List<int> buffer, int length)
+      native "StringBuffer_createStringFromUint16Array";
+
+  /**
+   * Creates a Uint16Array, as the scalar list.
+   *
+   * Used as a native here to avoid importing scalar lists.
+   */
+  static _Uint16Array _newBuffer(int length) native "Uint16Array_new";
 }