Fix some minor static type issues in dart:io implementation.

runtime/bin/process_impl.dart

Index: runtime/bin/process_impl.dart
diff --git a/runtime/bin/process_impl.dart b/runtime/bin/process_impl.dart
index 2fcdaabfc2ee5e64eb688fccdb64edf1c459a5f0..c068d2f2a5811f0c6646921b3d688372209bc5fc 100644
--- a/runtime/bin/process_impl.dart
+++ b/runtime/bin/process_impl.dart
@@ -197,9 +197,10 @@ class _Process implements Process {
   String _path;
   ObjectArray<String> _arguments;
   String _workingDirectory;
-  Socket _in;
-  Socket _out;
-  Socket _err;
+  // Private methods of _Socket are used by _in, _out, and _err.
+  _Socket _in;
+  _Socket _out;
+  _Socket _err;
   Socket _exitHandler;
   int _pid;
   bool _closed;