Add comment for the forked Dart revision for the assembler code.

src/assembler.cpp

 //===- subzero/src/assembler.cpp - Assembler base class -------------------===//
 // 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.
 //
 // Modified by the Subzero authors.
 //
+// This is forked from Dart revision 39313.
+// Please update the revision if we merge back changes from Dart.
+// https://code.google.com/p/dart/wiki/GettingTheSource
+//
 //===----------------------------------------------------------------------===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This file implements the Assembler class.
@@ skipping 122 matching lines @@
   }
   // Handle any bytes that are not prefixed by a fixup.
   for (intptr_t i = CurPosition; i < EndPosition; ++i) {
     Str << "\t.byte 0x";
     Str.write_hex(buffer_.Load<uint8_t>(i));
     Str << "\n";
   }
 }
 
 } // end of namespace Ice