Safely concatenate JavaScript

Most JavaScript packages wrap their code in IIFEs, which is good.

Pipeline used to concatenate JS only with a newline. That can break those IIFES
though:

    (function() {
      // package A
    }()) // No semicolon! Most people put one here, but unfortunately not all!
    (function() {
      // package B
    }());

The above is equivalent to:

    (function() {
      // package A
    }())(function() {
      // package B
    }());

Suddenly we have a function call!

With this commit, JS is concatenated with a newline followed by a semicolon,
which fixes the above issue:

    (function() {
      // package A
    }()) // No semicolon! Most people put one here, but unfortunately not all!
    ;(function() {
      // package B
    }());

There is no need to worry about superfluos semicolons, such as:

    (function() {
      // package A
    }());
    ;;(function() {
      // package B
    }());

That is still valid JavaScript and the extra semicolons will be removed by the
minifier.

pipeline/compressors/__init__.py

@@ -142,7 +142,11 @@ class Compressor(object):
 
     def concatenate(self, paths):
         """Concatenate together a list of files"""
-        return "\n".join([self.read_text(path) for path in paths])
+        # Note how a semicolon is added between the two files to make sure that
+        # their behavior is not changed. '(expression1)\n(expression2)' calls
+        # `expression1` with `expression2` as an argument! Superfluos semicolons
+        # are valid in JavaScript and will be removed by the minifier.
+        return "\n;".join([self.read_text(path) for path in paths])
 
     def construct_asset_path(self, asset_path, css_path, output_filename, variant=None):
         """Return a rewritten asset URL for a stylesheet"""

tests/assets/js/first.js

-function concat() {
+(function() {
+  window.concat = function() {
     console.log(arguments);
-}
+  }
+}()) // No semicolon

tests/assets/js/second.js

-function cat() {
+(function() {
+  window.cat = function() {
     console.log("hello world");
-}
+  }
+}());

tests/tests/test_compressor.py

@@ -46,7 +46,7 @@ class CompressorTest(TestCase):
             _('pipeline/js/first.js'),
             _('pipeline/js/second.js')
         ])
-        self.assertEqual("""function concat() {\n  console.log(arguments);\n}\n\nfunction cat() {\n  console.log("hello world");\n}\n""", js)
+        self.assertEqual("""(function() {\n  window.concat = function() {\n    console.log(arguments);\n  }\n}()) // No semicolon\n\n;(function() {\n  window.cat = function() {\n    console.log("hello world");\n  }\n}());\n""", js)
 
     @patch.object(base64, 'b64encode')
     def test_encoded_content(self, mock):