Add FileException.path and clean up file exceptions.

sdk/lib/io/file.dart

Index: sdk/lib/io/file.dart
diff --git a/sdk/lib/io/file.dart b/sdk/lib/io/file.dart
index 450a9020abb0920d6203897039b5173815366477..2115d31971a121ab22a0a4e61d86a71eb17624ee 100644
--- a/sdk/lib/io/file.dart
+++ b/sdk/lib/io/file.dart
@@ -525,21 +525,30 @@ abstract class RandomAccessFile {
 
 
 class FileException implements IOException {
+  final String message;
+  final String path;
+  final OSError osError;
   const FileException([String this.message = "",
-                         OSError this.osError = null]);
+                       String this.path = "",
+                       OSError this.osError]);
+
   String toString() {
     StringBuffer sb = new StringBuffer();
     sb.write("FileException");
     if (!message.isEmpty) {
       sb.write(": $message");
+      if (path != null) {
+        sb.write(", path = $path");
+      }
       if (osError != null) {
         sb.write(" ($osError)");
       }
     } else if (osError != null) {
       sb.write(": osError");
+      if (path != null) {
+        sb.write(", path = $path");
+      }
     }
     return sb.toString();
   }
-  final String message;
-  final OSError osError;
 }