Minor improvement to File diagnostics

dart/runtime/bin/file_impl.dart

Index: dart/runtime/bin/file_impl.dart
diff --git a/dart/runtime/bin/file_impl.dart b/dart/runtime/bin/file_impl.dart
index 145e4db197d8746f984d459fa2b03ddd78496ea8..2a94cace126e492011e331684c1bde619ad5594a 100644
--- a/dart/runtime/bin/file_impl.dart
+++ b/dart/runtime/bin/file_impl.dart
@@ -493,7 +493,12 @@ class _FileBase {
 // 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());