Generate homebrew script compatible with latest changes

Fixes #58

Change-Id: I23e38db5448c5d8ceb55d57689eda0eecf80c630
Reviewed-on: https://dart-review.googlesource.com/c/88883
Auto-Submit: Kevin Moore <kevmoo@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>
Commit-Queue: Alexander Thomas <athom@google.com>

lib/src/impl.dart

@@ -82,7 +82,7 @@ class Dart < Formula
   homepage "https://www.dartlang.org/"
 
   version "$stableVersion"
-  if MacOS.prefer_64_bit?
+  if Hardware::CPU.is_64_bit?
     url "$_urlBase/stable/release/${revisions['stable']}/$_x64File"
     sha256 "${hashes['stable'][_x64File]}"
   else
@@ -92,7 +92,7 @@ class Dart < Formula
 
   devel do
     version "$devVersion"
-    if MacOS.prefer_64_bit?
+    if Hardware::CPU.is_64_bit?
       url "$_urlBase/dev/release/${revisions['dev']}/$_x64File"
       sha256 "${hashes['dev'][_x64File]}"
     else