Add argument check to Directory and Link constructors

sdk/lib/io/link.dart

Index: sdk/lib/io/link.dart
diff --git a/sdk/lib/io/link.dart b/sdk/lib/io/link.dart
index 237154dc9a47db36a6e92d0645050cd960367b5d..8c28eb4e9ef2bcb4f5661145a62a6d6a17f21322 100644
--- a/sdk/lib/io/link.dart
+++ b/sdk/lib/io/link.dart
@@ -130,7 +130,13 @@ class _Link extends FileSystemEntity implements Link {
 
   SendPort _fileService;
 
-  _Link(String this.path);
+  _Link(String this.path) {
+    if (path is! String) {
+      throw new ArgumentError('${Error.safeToString(path)} '
+                              'is not a String');
+    }
+  }
+
 
   _Link.fromPath(Path inputPath) : path = inputPath.toNativePath();