brew install --ignore-dependencies

I used the same option flag as ruby gems does.

Library/Homebrew/ARGV+yeast.rb

@@ -82,11 +82,11 @@ module HomebrewArgvExtension
   def usage
     <<-EOS
 Usage: brew command [formula] ...
-Usage: brew [--prefix] [--cache] [--version]
+Usage: brew [--prefix] [--cache] [--version|-v]
 Usage: brew [--verbose|-v]
 
 Commands:
-  install formula ... [--debug|-d] [--interactive|-i]
+  install formula ... [--debug|-d] [--interactive|-i] [--ignore-dependencies]
   remove formula ...
   search [regex]
   list [formula] ...

bin/brew

@@ -107,9 +107,11 @@ begin
         formulae = ARGV.formulae
       end
 
-      deps = []
-      formulae.each { |f| deps += expand_deps f }
-      formulae = deps.reject { |f| f.installed? }
+      unless ARGV.include? '--ignore-dependencies'
+        deps = []
+        formulae.each { |f| deps += expand_deps f }
+        formulae = deps.reject { |f| f.installed? }
+      end
 
       require 'set'
       done = Set.new