Avoid multiple reallocations of StringBuffer._parts

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 {
   static const int _BUFFER_SIZE = 64;
   static const int _PARTS_TO_COMPACT = 128;
   static const int _PARTS_TO_COMPACT_SIZE_LIMIT = _PARTS_TO_COMPACT * 8;
 
   /**
@@ skipping 130 matching lines @@
   /**
    * Adds a new part to this string buffer and keeps track of how
    * many code units are contained in the parts.
    */
   void _addPart(String str) {
     int length = str.length;
     _partsCodeUnits += length;
     _partsCodeUnitsSinceCompaction += length;
 
     if (_parts == null) {
-      _parts = [ str ];
+      // Empirically this is a good capacity to minimize total bytes allocated.
+      _parts = new _GrowableList.withCapacity(10)..add(str);
     } else {
       _parts.add(str);
       int partsSinceCompaction = _parts.length - _partsCompactionIndex;
       if (partsSinceCompaction == _PARTS_TO_COMPACT) {
         _compact();
       }
     }
   }
 
   /**
@@ skipping 13 matching lines @@
     _partsCodeUnitsSinceCompaction = 0;
     _partsCompactionIndex = _parts.length;
   }
 
   /**
    * Create a [String] from the UFT-16 code units in buffer.
    */
   static String _create(Uint16List buffer, int length, bool isLatin1)
       native "StringBuffer_createStringFromUint16Array";
 }