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)} '
+                              'is not a String');
+    }
+  }
 
   // Constructor from Path for file.
   _File.fromPath(Path path) : this(path.toNativePath());