VM: Remove explicit field initialization with null from library classes.

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;
 
   /**
    * When strings are written to the string buffer, we add them to a
    * list of string parts.
    */
-  List<String> _parts = null;
+  List<String> _parts;
 
   /**
     * Total number of code units in the string parts. Does not include
     * the code units added to the buffer.
     */
   int _partsCodeUnits = 0;
 
   /**
    * To preserve memory, we sometimes compact the parts. This combines
    * several smaller parts into a single larger part to cut down on the
    * cost that comes from the per-object memory overhead. We keep track
    * of the last index where we ended our compaction and the number of
    * code units added since the last compaction.
    */
   int _partsCompactionIndex = 0;
   int _partsCodeUnitsSinceCompaction = 0;
 
   /**
    * The buffer is used to build up a string from code units. It is
    * used when writing short strings or individual char codes to the
    * buffer. The buffer is allocated on demand.
    */
-  Uint16List _buffer = null;
+  Uint16List _buffer;
   int _bufferPosition = 0;
 
   /**
    * Collects the approximate maximal magnitude of the code units added
    * to the buffer.
    *
    * The value of each added code unit is or'ed with this variable, so the
    * most significant bit set in any code unit is also set in this value.
    * If below 256, the string in the buffer is a Latin-1 string.
    */
@@ skipping 116 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";
 }