Add support for building the Dart VM for Android OS.

runtime/bin/platform_android.cc

Index: runtime/bin/platform_android.cc
diff --git a/runtime/bin/platform_macos.cc b/runtime/bin/platform_android.cc
similarity index 90%
copy from runtime/bin/platform_macos.cc
copy to runtime/bin/platform_android.cc
index 81455ba3d3988ae7348433d517836d4b83fb96ed..8d9bcf76154f8fb907994331ca92371c7fc5c016 100644
--- a/runtime/bin/platform_macos.cc
+++ b/runtime/bin/platform_android.cc
@@ -4,7 +4,6 @@
 
 #include "bin/platform.h"
 
-#include <crt_externs.h>
 #include <signal.h>
 #include <string.h>
 #include <unistd.h>
@@ -31,7 +30,7 @@ int Platform::NumberOfProcessors() {
 
 
 const char* Platform::OperatingSystem() {
-  return "macos";
+  return "android";
 }
 
 
@@ -43,9 +42,6 @@ bool Platform::LocalHostname(char *buffer, intptr_t buffer_length) {
 char** Platform::Environment(intptr_t* count) {
   // Using environ directly is only safe as long as we do not
   // provide access to modifying environment variables.
-  // On MacOS you have to do a bit of magic to get to the
-  // environment strings.
-  char** environ = *(_NSGetEnviron());
   intptr_t i = 0;
   char** tmp = environ;
   while (*(tmp++) != NULL) i++;