Rip out the last dart:io dependency from pkg/http.

pkg/http/lib/src/multipart_file.dart

Index: pkg/http/lib/src/multipart_file.dart
diff --git a/pkg/http/lib/src/multipart_file.dart b/pkg/http/lib/src/multipart_file.dart
index 220c28633f4cb59b7e63817e3a7fd6d6c614096a..07758b9097fb5713eebfed084c0643c7da3a2b7f 100644
--- a/pkg/http/lib/src/multipart_file.dart
+++ b/pkg/http/lib/src/multipart_file.dart
@@ -5,7 +5,6 @@
 library multipart_file;
 
 import 'dart:async';
-import 'dart:io';
 import 'dart:convert';
 
 import 'package:http_parser/http_parser.dart';
@@ -13,6 +12,7 @@ import 'package:path/path.dart' as path;
 import 'package:stack_trace/stack_trace.dart';
 
 import 'byte_stream.dart';
+import 'io.dart' as io;
 import 'utils.dart';
 
 /// A file to be uploaded as part of a [MultipartRequest]. This doesn't need to
@@ -86,10 +86,13 @@ class MultipartFile {
   /// [filename] defaults to the basename of [filePath]. [contentType] currently
   /// defaults to `application/octet-stream`, but in the future may be inferred
   /// from [filename].
+  ///
+  /// This can only be used in an environment that supports "dart:io".
   static Future<MultipartFile> fromPath(String field, String filePath,
       {String filename, MediaType contentType}) {
+    io.assertSupported("MultipartFile.fromPath");
     if (filename == null) filename = path.basename(filePath);
-    var file = new File(filePath);
+    var file = io.newFile(filePath);
     return Chain.track(file.length()).then((length) {
       var stream = new ByteStream(Chain.track(file.openRead()));
       return new MultipartFile(field, stream, length,