pkg/utf: fixed layout, added todos, updated docs and homepage pubspec links

pkg/utf/lib/src/constants.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.
 
-part of utf;
+library utf.constants;
 
 /**
  * Invalid codepoints or encodings may be substituted with the value U+fffd.
  */
 const int UNICODE_REPLACEMENT_CHARACTER_CODEPOINT = 0xfffd;
 const int UNICODE_BOM = 0xfeff;
 const int UNICODE_UTF_BOM_LO = 0xff;
 const int UNICODE_UTF_BOM_HI = 0xfe;
 
 const int UNICODE_BYTE_ZERO_MASK = 0xff;
 const int UNICODE_BYTE_ONE_MASK = 0xff00;
 const int UNICODE_VALID_RANGE_MAX = 0x10ffff;
 const int UNICODE_PLANE_ONE_MAX = 0xffff;
 const int UNICODE_UTF16_RESERVED_LO = 0xd800;
 const int UNICODE_UTF16_RESERVED_HI = 0xdfff;
 const int UNICODE_UTF16_OFFSET = 0x10000;
 const int UNICODE_UTF16_SURROGATE_UNIT_0_BASE = 0xd800;
 const int UNICODE_UTF16_SURROGATE_UNIT_1_BASE = 0xdc00;
 const int UNICODE_UTF16_HI_MASK = 0xffc00;
 const int UNICODE_UTF16_LO_MASK = 0x3ff;