Use JsShrink if jsxmin is not available

Summary: If `jsxmin` is not available, use a pure PHP implementation instead (JsShrink).

Test Plan:
  - Ran `arc lint --lintall` on all JS and fixed every relevant warning.
  - Forced minification on and browsed around the site using JS behaviors. Didn't hit anything problematic.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5670

externals/JsShrink/jsShrink.php

@@ -0,0 +1,45 @@
+<?php
+/** Remove spaces and comments from JavaScript code
+* @param string code with commands terminated by semicolon
+* @return string shrinked code
+* @link http://vrana.github.com/JsShrink/
+* @author Jakub Vrana, http://www.vrana.cz/
+* @copyright 2007 Jakub Vrana
+* @license http://www.apache.org/licenses/LICENSE-2.0 Apache License, Version 2.0
+* @license http://www.gnu.org/licenses/gpl-2.0.html GNU General Public License, version 2 (one or other)
+*/
+function jsShrink($input) {
+	return preg_replace_callback('(
+		(?:
+			(^|[-+\([{}=,:;!%^&*|?~]|/(?![/*])|return|throw) # context before regexp
+			(?:\s|//[^\n]*+\n|/\*(?:[^*]|\*(?!/))*+\*/)* # optional space
+			(/(?![/*])(?:\\\\[^\n]|[^[\n/\\\\]|\[(?:\\\\[^\n]|[^]])++)+/) # regexp
+			|(^
+				|\'(?:\\\\.|[^\n\'\\\\])*\'
+				|"(?:\\\\.|[^\n"\\\\])*"
+				|([0-9A-Za-z_$]+)
+				|([-+]+)
+				|.
+			)
+		)(?:\s|//[^\n]*+\n|/\*(?:[^*]|\*(?!/))*+\*/)* # optional space
+	)sx', 'jsShrinkCallback', "$input\n");
+}
+
+function jsShrinkCallback($match) {
+	static $last = '';
+	$match += array_fill(1, 5, null); // avoid E_NOTICE
+	list(, $context, $regexp, $result, $word, $operator) = $match;
+	if ($word != '') {
+		$result = ($last == 'word' ? "\n" : ($last == 'return' ? " " : "")) . $result;
+		$last = ($word == 'return' || $word == 'throw' || $word == 'break' ? 'return' : 'word');
+	} elseif ($operator) {
+		$result = ($last == $operator[0] ? "\n" : "") . $result;
+		$last = $operator[0];
+	} else {
+		if ($regexp) {
+			$result = $context . ($context == '/' ? "\n" : "") . $regexp;
+		}
+		$last = '';
+	}
+	return $result;
+}

externals/JsShrink/readme.txt

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+JsShrink - Remove spaces and comments from JavaScript code
+Available in PHP and JavaScript
+Requires statements ending by semicolon, use JSHint or JSLint to verify.
+
+http://vrana.github.com/JsShrink/
+
+Usage PHP:
+<?php
+include "jsShrink.php";
+echo jsShrink($code);
+?>
+
+Usage JavaScript:
+<script type="text/javascript" src="jsShrink.js"></script>
+<script type="text/javascript">
+textarea.value = jsShrink(code);
+</script>
+
+Note:
+Google Closure Compiler is much more powerful and efficient tool.
+JsShrink was created for those looking for PHP or JavaScript only solution.
+Most other JS minifiers are not able to process valid JavaScript code:
+http://php.vrana.cz/minifikace-javascriptu.php#srovnani