Remove check for scheme in source-loader.

pkg/front_end/lib/physical_file_system.dart

 // Copyright (c) 2016, 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.
 
 library front_end.physical_file_system;
 
 import 'dart:async';
 import 'dart:io' as io;
 
 import 'file_system.dart';
 
 /// Concrete implementation of [FileSystem] which performs its operations using
 /// I/O.
 ///
 /// Not intended to be implemented or extended by clients.
 class PhysicalFileSystem implements FileSystem {
   static final PhysicalFileSystem instance = new PhysicalFileSystem._();
 
   PhysicalFileSystem._();
 
   @override
   FileSystemEntity entityForUri(Uri uri) {
     if (uri.scheme != 'file' && uri.scheme != '') {
-      throw new ArgumentError('File URI expected');
+      throw new FileSystemException(
+          uri, 'PhysicalFileSystem only supports file:* URIs');
     }
     return new _PhysicalFileSystemEntity(Uri.base.resolveUri(uri));
   }
 }
 
 /// Concrete implementation of [FileSystemEntity] for use by
 /// [PhysicalFileSystem].
 class _PhysicalFileSystemEntity implements FileSystemEntity {
   @override
   final Uri uri;
@@ skipping 39 matching lines @@
    */
   FileSystemException _toFileSystemException(io.FileSystemException exception) {
     String message = exception.message;
     String osMessage = exception.osError?.message;
     if (osMessage != null && osMessage.isNotEmpty) {
       message = osMessage;
     }
     return new FileSystemException(uri, message);
   }
 }