Improve error handling in the process library.

runtime/bin/process_impl.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 class _ProcessStartStatus {
   int _errorCode;  // Set to OS error code if process start failed.
   String _errorMessage;  // Set to OS error message if process start failed.
 }
 
 
 class _Process implements Process {
 
   _Process(String path, List<String> arguments) {
+    if (path is !String) {
+      throw new ProcessException("Path is not a String: $path");
+    }
     _path = path;
-    {
-      int len = arguments.length;
-      _arguments = new ObjectArray<String>(len);
-      for (int i = 0; i < len; i++) {
-        _arguments[i] = arguments[i];
+
+    if (arguments is !List) {
+      throw new ProcessException("Arguments is not a List: $arguments");
+    }
+    int len = arguments.length;
+    _arguments = new ObjectArray<String>(len);
+    for (int i = 0; i < len; i++) {
+      var arg = arguments[i];
+      if (arg is !String) {
+        throw new ProcessException("Non-string argument: $arg");
       }
+      _arguments[i] = arguments[i];
     }
+
     _in = new _Socket._internal();
     _out = new _Socket._internal();
     _err = new _Socket._internal();
     _exitHandler = new _Socket._internal();
     _closed = false;
     _killed = false;
     _started = false;
     _exitHandlerCallback = null;
   }
 
@@ skipping 112 matching lines @@
   Socket _in;
   Socket _out;
   Socket _err;
   Socket _exitHandler;
   int _pid;
   bool _closed;
   bool _killed;
   bool _started;
   var _exitHandlerCallback;
 }