Add new HttpMultipartFormData, used for parsing a MimeMultipart and extracting either text or binar…

tests/standalone/io/mime_multipart_parser_test.dart

Index: tests/standalone/io/mime_multipart_parser_test.dart
diff --git a/tests/standalone/io/mime_multipart_parser_test.dart b/tests/standalone/io/mime_multipart_parser_test.dart
index 1045b5a0f18abdd6665bd198dfa136e4aa7f1917..5a81df74a1eadc591d0abef42e5aee4115f43617 100644
--- a/tests/standalone/io/mime_multipart_parser_test.dart
+++ b/tests/standalone/io/mime_multipart_parser_test.dart
@@ -279,12 +279,8 @@ Content-Type: text/plain\r
 --\r\r\r
 -\r""";
   testParse(message, "boundary", [headers, headers], [body1, body2]);
-}
-
-void testParseInvalid() {
-  String message;
 
-  // Missing initial CRLF. One body part less.
+  // Without initial CRLF.
   message = """
 --xxx\r
 \r
@@ -295,7 +291,11 @@ Body 1\r
 \r
 Body2\r
 --xxx--\r\n""";
-  testParse(message, "xxx", null, ["\r\nBody2"]);
+  testParse(message, "xxx", null, ["\r\nBody 1", "\r\nBody2"]);
+}
+
+void testParseInvalid() {
+  String message;
 
   // Missing end boundary.
   message = """