Include command in ProcessException.

sdk/lib/io/process.dart

Index: sdk/lib/io/process.dart
diff --git a/sdk/lib/io/process.dart b/sdk/lib/io/process.dart
index 8d6d9f23026f2ea92fd31bfd8e7626a98eebb113..093ed6a18867415fbf963f2f0c594e2784f52221 100644
--- a/sdk/lib/io/process.dart
+++ b/sdk/lib/io/process.dart
@@ -214,8 +214,18 @@ class ProcessSignal {
 
 
 class ProcessException implements Exception {
-  const ProcessException([String this.message = "", int this.errorCode = 0]);
-  String toString() => "ProcessException: $message ($errorCode)";
+  const ProcessException([String this.command = "",
+                          String this.message = "",
+                          int this.errorCode = 0]);
+  String toString() {
+    var msg = (message == null) ? 'OS error code: $errorCode' : message;
+    return "ProcessException: $msg\n  Command: $command";
+  }
+
+  /**
+   * Contains the command for this process.
+   */
+  final String command;
 
   /**
    * Contains the system message for the process exception if any.