[dart:io] Make Platform.localeName fall back on en_US if LANG is not defined

runtime/bin/platform_fuchsia.cc

 // Copyright (c) 2016, 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 "platform/globals.h"
 #if defined(HOST_OS_FUCHSIA)
 
 #include "bin/platform.h"
 
 #include <string.h>  // NOLINT
@@ skipping 30 matching lines @@
   return "lib";
 }
 
 
 const char* Platform::LibraryExtension() {
   return "so";
 }
 
 
 const char* Platform::LocaleName() {
-  return getenv("LANG");
+  char* lang = getenv("LANG");
+  if (lang == NULL) {
+    return "en_US";
+  }
+  return lang;
 }
 
 
 bool Platform::LocalHostname(char* buffer, intptr_t buffer_length) {
   return gethostname(buffer, buffer_length) == 0;
 }
 
 
 char** Platform::Environment(intptr_t* count) {
   // Using environ directly is only safe as long as we do not
@@ skipping 53 matching lines @@
 
 
 void Platform::Exit(int exit_code) {
   exit(exit_code);
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(HOST_OS_FUCHSIA)