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

runtime/bin/platform_android.cc

Index: runtime/bin/platform_android.cc
diff --git a/runtime/bin/platform_android.cc b/runtime/bin/platform_android.cc
index cba7fa497e28561a0667c6eefabb1de84a361559..b72e3b8b14b65d74a0c10d9daecbf8a84620ad18 100644
--- a/runtime/bin/platform_android.cc
+++ b/runtime/bin/platform_android.cc
@@ -58,7 +58,11 @@ const char* Platform::LibraryExtension() {
 
 
 const char* Platform::LocaleName() {
-  return getenv("LANG");
+  char* lang = getenv("LANG");
+  if (lang == NULL) {
+    return "en_US";
+  }
+  return lang;
 }