Remove check for scheme in source-loader.

pkg/front_end/lib/physical_file_system.dart

Index: pkg/front_end/lib/physical_file_system.dart
diff --git a/pkg/front_end/lib/physical_file_system.dart b/pkg/front_end/lib/physical_file_system.dart
index 2fe685619524c6ebea049d110332ebef42263932..083f802de7da9bae2d4d1ca5754b000c39e1d622 100644
--- a/pkg/front_end/lib/physical_file_system.dart
+++ b/pkg/front_end/lib/physical_file_system.dart
@@ -21,7 +21,8 @@ class PhysicalFileSystem implements FileSystem {
   @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));
   }