Add rake task to build with closure advanced optimizations.

You would use this to compile your code _with_ underscore, to
remove all those dead underscore functions you don't use.

Slight tweak to code to `root._` code to get working, and must
remove anonymous wrapper function for current closure compiler to
remove dead code.

Rakefile

@@ -1,9 +1,27 @@
 require 'rubygems'
 require 'closure-compiler'
-
+  
 desc "Use the Closure Compiler to compress Underscore.js"
 task :build do
   js  = File.open('underscore.js', 'r')
   min = Closure::Compiler.new.compile(js)
   File.open('underscore-min.js', 'w') {|f| f.write(min) }
-end
+end
+
+task :build_advanced do
+  js  = File.read('underscore.js')
+  # remove wrapping anonymous function as this messes with closure compiler
+  # see
+  # http://groups.google.com/group/closure-compiler-discuss/browse_thread/thread/b59b54c1a0073aa5
+  js.sub!('(function() {', '').chomp!("})();\n")
+  compiler = Closure::Compiler.new \
+    :compilation_level => 'ADVANCED_OPTIMIZATIONS', 
+    :formatting => 'PRETTY_PRINT'
+  min = compiler.compile(js)
+  File.open('underscore-min2.js', 'w') {|f| f.write(min) }
+  #
+  original_size = js.length
+  minimized_size = min.length
+  puts original_size, minimized_size
+end
+

underscore.js

@@ -25,8 +25,9 @@
   var breaker = typeof StopIteration !== 'undefined' ? StopIteration : '__break__';
 
   // Create a safe reference to the Underscore object for reference below.
-  var _ = root._ = function(obj) { return new wrapper(obj); };
-
+  var _ = function(obj) { return new wrapper(obj); };
+  root._ = _;
+  
   // Export the Underscore object for CommonJS.
   if (typeof exports !== 'undefined') exports._ = _;