Add optional workingDirectory argument to Process.start.

runtime/bin/process_impl.dart

Index: runtime/bin/process_impl.dart
===================================================================
--- runtime/bin/process_impl.dart	(revision 2981)
+++ runtime/bin/process_impl.dart	(working copy)
@@ -10,7 +10,9 @@
 
 class _Process implements Process {
 
-  _Process.start(String path, List<String> arguments) {
+  _Process.start(String path, 
+                 List<String> arguments, 
+                 [String workingDirectory]) {
     if (path is !String) {
       throw new ProcessException("Path is not a String: $path");
     }
@@ -29,6 +31,12 @@
       _arguments[i] = arguments[i];
     }
 
+    if (workingDirectory is !String && workingDirectory !== null) {
+      throw new ProcessException(
+          "WorkingDirectory is not a String: $workingDirectory");
+    }
+    _workingDirectory = workingDirectory;
+
     _in = new _Socket._internalReadOnly();  // stdout coming from process.
     _out = new _Socket._internalWriteOnly();  // stdin going to process.
     _err = new _Socket._internalReadOnly();  // stderr coming from process.
@@ -51,8 +59,14 @@
 
   void start() {
     var status = new _ProcessStartStatus();
-    bool success = _start(
-        _path, _arguments, _in, _out, _err, _exitHandler, status);
+    bool success = _start(_path, 
+                          _arguments, 
+                          _workingDirectory, 
+                          _in, 
+                          _out, 
+                          _err, 
+                          _exitHandler, 
+                          status);
     if (!success) {
       close();
       if (_errorHandler !== null) {
@@ -106,6 +120,7 @@
 
   bool _start(String path,
               List<String> arguments,
+              String workingDirectory,
               Socket input,
               Socket output,
               Socket error,
@@ -187,6 +202,7 @@
 
   String _path;
   ObjectArray<String> _arguments;
+  String _workingDirectory;
   Socket _in;
   Socket _out;
   Socket _err;