Add a multipart/form-data parser

[nex3]

The pkg/http library includes code to generate multipart/form-data requests, but there's no code in Dart to parse such requests. This is necessary to handle file uploads, or to fully test code that performs file uploads.

This could either go in dart:io somewhere, or in a separate package.

[sgjesse]

A push based multipart/form-data body decoder is present in sdk/lib/io/mime_multipart_parser.dart with some tests in tests/standalone/io/mime_multipart_parser_test.dart. This needs some more care and some library to expose it.

[sgjesse]

Added Duplicate label.
Marked as being merged into #2488.