Minor improvement to File diagnostics

dart/runtime/bin/file_impl.dart

 // Copyright (c) 2012, 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.
 
 class _FileInputStream extends _BaseDataInputStream implements InputStream {
   _FileInputStream(String name)
       : _data = const [],
         _position = 0,
         _filePosition = 0 {
     var file = new File(name);
@@ skipping 475 matching lines @@
         return new FileIOException("File closed");
       default:
         return new Exception("Unknown error");
     }
   }
 }
 
 // Class for encapsulating the native implementation of files.
 class _File extends _FileBase implements File {
   // Constructor for file.
-  _File(String this._name);
+  _File(String this._name) {
+    if (_name is! String) {
+      throw new ArgumentError('${NoSuchMethodError.safeToString(_name)} '

[Ivan Posva, 2012/10/02 00:19:38]

Peter can you explain why you are using NoSuchMethodError.safeToString here? I
understand how it can make sense in NoSuchMethodError handling, but here not so.

[ahe, 2012/10/02 04:49:24]

It makes sense to use safeToString here because the error is about the type of
the object. Generally, calling toString during error reporting is undesirable
for the same reasons as in NoSuchMethodError. 

[Ivan Posva, 2012/10/02 05:19:11]

It just feels very strange to be calling some random static method of some
exception here.

[ahe, 2012/10/02 05:39:47]

Agreed, I would like to find a better place for the static method, and perhaps
make it more convenient to use. I'll file a bug. 

[ahe, 2012/10/02 07:47:11]

Filed http://dartbug.com/5596.

+                              'is not a String');
+    }
+  }
 
   // Constructor from Path for file.
   _File.fromPath(Path path) : this(path.toNativePath());
 
   Future<bool> exists() {
     _ensureFileService();
     List request = new List(2);
     request[0] = _FileUtils.EXISTS_REQUEST;
     request[1] = _name;
     return _fileService.call(request).transform((response) {
@@ skipping 658 matching lines @@
           new FileIOException("File closed '$_name'"));
     });
     return completer.future;
   }
 
   final String _name;
   int _id;
 
   SendPort _fileService;
 }