Adding library level docs for async, io, convert and fixing a bug in dart:core

sdk/lib/convert/convert.dart

 // Copyright (c) 2013, 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.
 
 /**
- * Converters for JSON and UTF-8, as well as support for creating additional
- * converters.
+ *
+ * Encoders and decoders for converting between different data representations,
+ * including JSON and UTF-8.
+ *
+ * In addition to converters for common data representations, this library
+ * provides support for implementing converters in a way which makes them easy to
+ * chain and to use with streams.
+ *
+ * The `dart:convert` library works in both web apps and command-line apps.
+ * To use it:
+ *
+ *     import 'dart:convert';
+ *
+ * Two commonly used converters are the top-level instances of
+ * [JsonCodec] and [Utf8Codec], named JSON and UTF8, respectively.
+ *
+ * JSON is a simple text format for representing
+ * structured objects and collections.
+ * The JSON encoder/decoder transforms between strings and
+ * object structures, such as lists and maps, using the JSON format.
+ *
+ * UTF-8 is a common variable-width encoding that can represent
+ * every character in the Unicode character set.
+ * The UTF-8 encoder/decoder transforms between Strings and bytes.
+ *
+ * Converters are often used with streams
+ * to transform the data that comes through the stream
+ * as it becomes available.
+ * The following code uses two converters.
+ * The first is a UTF-8 decoder, which converts the data from bytes to UTF-8
+ * as it's read from a file,
+ * The second is an instance of [LineSplitter],
+ * which splits the data on newline boundaries.
+ *
+ *     int lineNumber = 1;
+ *     Stream<List<int>> stream = new File('quotes.txt').openRead();
+ *
+ *     stream.transform(UTF8.decoder)
+ *           .transform(const LineSplitter())
+ *           .listen((line) {
+ *             if (showLineNumbers) {
+ *               stdout.write('${lineNumber++} ');
+ *             }
+ *             stdout.writeln(line);
+ *           });
+ *
+ * See the documentation for the [Codec] and [Converter] classes
+ * for information about creating your own converters.
  */
 library dart.convert;
 
 import 'dart:async';
 import "dart:collection" show HashSet;
 
 part 'ascii.dart';
 part 'byte_conversion.dart';
 part 'chunked_conversion.dart';
 part 'codec.dart';
 part 'converter.dart';
 part 'encoding.dart';
 part 'html_escape.dart';
 part 'json.dart';
 part 'latin1.dart';
 part 'line_splitter.dart';
 part 'string_conversion.dart';
 part 'utf.dart';