Add documentation to the FileSystemException class

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 d48a367005e572cd5daaf9959f83ff27c02e89ac..9bba4be19ca353e5bc6e62afbe673bdfb370dba4 100644
--- a/sdk/lib/io/file.dart
+++ b/sdk/lib/io/file.dart
@@ -561,10 +561,34 @@ abstract class RandomAccessFile {
 }
 
 
+/**
+ * Exception thrown when a file operation fails.
+ */
 class FileSystemException implements IOException {
+  /**
+   * Message describing the error. This does not include any detailed
+   * information form the underlying OS error. Check [osError] for
+   * that information.
+   */
   final String message;
+
+  /**
+   * The file system path on which the error occurred. Can be `null`
+   * if the exception does not relate directly to a file system path.
+   */
   final String path;
+
+  /**
+   * The underlying OS error. Can be `null` if the exception is not
+   * raised due to an OS error.
+   */
   final OSError osError;
+
+  /**
+   * Creates a new FileSystemException with an optional error message
+   * [message], optional file system path [path] and optional OS error
+   * [osError].
+   */
   const FileSystemException([this.message = "", this.path = "", this.osError]);
 
   String toString() {
@@ -573,16 +597,18 @@ class FileSystemException implements IOException {
     if (!message.isEmpty) {
       sb.write(": $message");
       if (path != null) {
-        sb.write(", path = $path");
+        sb.write(", path = '$path'");
       }
       if (osError != null) {
         sb.write(" ($osError)");
       }
     } else if (osError != null) {
-      sb.write(": osError");
+      sb.write(": $osError");
       if (path != null) {
-        sb.write(", path = $path");
+        sb.write(", path = '$path'");
       }
+    } else if (path != null) {
+      sb.write(": $path");
     }
     return sb.toString();
   }