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

LANG is not defined on Fuchsia buildbots, causing some build-breakage.

R=pylaligand@google.com

Review URL: https://codereview.chromium.org/2843343002 .

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;
 }
 
 

runtime/bin/platform_fuchsia.cc

@@ -48,7 +48,11 @@ const char* Platform::LibraryExtension() {
 
 
 const char* Platform::LocaleName() {
-  return getenv("LANG");
+  char* lang = getenv("LANG");
+  if (lang == NULL) {
+    return "en_US";
+  }
+  return lang;
 }
 
 

runtime/bin/platform_linux.cc

@@ -83,7 +83,11 @@ const char* Platform::LibraryExtension() {
 
 
 const char* Platform::LocaleName() {
-  return getenv("LANG");
+  char* lang = getenv("LANG");
+  if (lang == NULL) {
+    return "en_US";
+  }
+  return lang;
 }