Refactor the process creation code on POSIX platforms

runtime/bin/process.cc

 // Copyright (c) 2013, 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.
 
 #include "bin/dartutils.h"
 #include "bin/io_buffer.h"
 #include "bin/platform.h"
 #include "bin/process.h"
 #include "bin/socket.h"
 #include "bin/utils.h"
@@ skipping 133 matching lines @@
       Socket::SetSocketIdNativeField(stdin_handle, process_stdin);
       Socket::SetSocketIdNativeField(stdout_handle, process_stdout);
       Socket::SetSocketIdNativeField(stderr_handle, process_stderr);
       Socket::SetSocketIdNativeField(exit_handle, exit_event);
     }
     Process::SetProcessIdNativeField(process, pid);
   } else {
     DartUtils::SetIntegerField(
         status_handle, "_errorCode", error_code);
     DartUtils::SetStringField(
-        status_handle, "_errorMessage", os_error_message);
+        status_handle,
+        "_errorMessage",
+        os_error_message != NULL ? os_error_message
+                                 : "Cannot get error message");
   }
   delete[] string_args;
   delete[] string_environment;
   free(os_error_message);
   Dart_SetReturnValue(args, Dart_NewBoolean(error_code == 0));
 }
 
 
 void FUNCTION_NAME(Process_Wait)(Dart_NativeArguments args) {
   Dart_Handle process =  Dart_GetNativeArgument(args, 0);
@@ skipping 143 matching lines @@
   int external_length = strlen(system_string);
   uint8_t* buffer = NULL;
   Dart_Handle external_array = IOBuffer::Allocate(external_length, &buffer);
   memmove(buffer, system_string, external_length);
   if (utf8 != system_string) free(const_cast<char*>(system_string));
   Dart_SetReturnValue(args, external_array);
 }
 
 }  // namespace bin
 }  // namespace dart