only minify browser build if you have uglifyjs installed

Cakefile

task 'build:browser', 'rebuild the merged script for inclusion in the browser', ->
  code = ''
  for name in ['helpers', 'rewriter', 'lexer', 'parser', 'scope', 'nodes', 'coffee-script', 'browser']
    code += """
      require['./#{name}'] = new function() {
        var exports = this;
        #{fs.readFileSync "lib/#{name}.js"}
      };
    """
  code = """
    this.CoffeeScript = function() {
      function require(path){ return require[path]; }
      #{code}
      return require['./coffee-script']
    }()
  """
  try
    {parser, uglify} = require 'uglify-js'
    ast = parser.parse code
    code = uglify.gen_code uglify.ast_squeeze uglify.ast_mangle ast, extra: yes
  fs.writeFileSync 'extras/coffee-script.js', header + '\n' + code
  invoke 'test:browser'

gistfile2.diff

diff --git a/Cakefile b/Cakefile
index e656699..231c6e3 100644
--- a/Cakefile
+++ b/Cakefile
@@ -94,15 +94,17 @@ task 'build:browser', 'rebuild the merged script for inclusion in the browser',
         #{fs.readFileSync "lib/#{name}.js"}
       };
     """
-  {parser, uglify} = require 'uglify-js'
-  ast = parser.parse """
+  code = """
     this.CoffeeScript = function() {
       function require(path){ return require[path]; }
       #{code}
       return require['./coffee-script']
     }()
   """
-  code = uglify.gen_code uglify.ast_squeeze uglify.ast_mangle ast, extra: yes
+  try
+    {parser, uglify} = require 'uglify-js'
+    ast = parser.parse code
+    code = uglify.gen_code uglify.ast_squeeze uglify.ast_mangle ast, extra: yes
   fs.writeFileSync 'extras/coffee-script.js', header + '\n' + code
   invoke 'test:browser'