Change process exit codes on Windows to always be positive

runtime/bin/process_win.cc

Index: runtime/bin/process_win.cc
diff --git a/runtime/bin/process_win.cc b/runtime/bin/process_win.cc
index 00514c775a60d437ea6a69daae7ce44ada2dab59..6247ac946f1ea39439efcdb1f2b2a24e44e34d15 100644
--- a/runtime/bin/process_win.cc
+++ b/runtime/bin/process_win.cc
@@ -155,18 +155,13 @@ class ProcessInfoList {
       FATAL("Failed unregistering wait operation");
     }
     // Get and report the exit code to Dart.
-    int exit_code;
+    DWORD exit_code;
     ok = GetExitCodeProcess(handle,
                             reinterpret_cast<DWORD*>(&exit_code));
     if (!ok) {
       FATAL1("GetExitCodeProcess failed %d\n", GetLastError());
     }
-    int negative = 0;
-    if (exit_code < 0) {
-      exit_code = abs(exit_code);
-      negative = 1;
-    }
-    int message[2] = { exit_code, negative };
+    int message[2] = { exit_code, 0 };
     DWORD written;
     ok = WriteFile(exit_pipe, message, sizeof(message), &written, NULL);
     // If the process has been closed, the read end of the exit