diff --git a/.gitignore b/.gitignore
index ca2c1a9..f3f3bdf 100644
--- a/.gitignore
+++ b/.gitignore
@@ -8,3 +8,4 @@
 /composer.lock
 /vendor
 /.php_cs.cache
+/tests/compiler.jar
diff --git a/.travis.yml b/.travis.yml
index e1a2519..359e8e2 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -25,8 +25,14 @@ cache:
 install:
   - composer update --no-interaction --prefer-source
 
+before_script:
+  - wget -c https://dl.google.com/closure-compiler/compiler-latest.zip -O vendor/compiler-latest.zip
+  - unzip -fd vendor/closure-compiler vendor/compiler-latest.zip
+  - ln -sfn ../$(echo vendor/closure-compiler/closure-compiler-*.jar) tests/compiler.jar
+  - java -jar tests/compiler.jar --version
+
 script:
   - composer validate
-  - phpunit
+  - phpunit --verbose
 
 # vim:ts=2:sw=2:et
diff --git a/lib/Minify/ClosureCompiler.php b/lib/Minify/ClosureCompiler.php
index 487cafe..26cbc67 100644
--- a/lib/Minify/ClosureCompiler.php
+++ b/lib/Minify/ClosureCompiler.php
@@ -24,19 +24,12 @@
  *
  * </code>
  *
- * @todo unit tests, $options docs
- * @todo more options support (or should just passthru them all?)
- *
  * @package Minify
  * @author Stephen Clay <steve@mrclay.org>
  * @author Elan Ruusamäe <glen@delfi.ee>
  */
 class Minify_ClosureCompiler
 {
-    const OPTION_CHARSET = 'charset';
-    const OPTION_COMPILATION_LEVEL = 'compilation_level';
-    const OPTION_WARNING_LEVEL = 'warning_level';
-
     public static $isDebug = false;
 
     /**
@@ -61,6 +54,17 @@ class Minify_ClosureCompiler
      */
     public static $javaExecutable = 'java';
 
+    /**
+     * Default command line options passed to closure-compiler
+     *
+     * @var array
+     */
+    public static $defaultOptions = array(
+        'charset' => 'utf-8',
+        'compilation_level' => 'SIMPLE_OPTIMIZATIONS',
+        'warning_level' => 'QUIET',
+    );
+
     /**
      * Minify a Javascript string
      *
@@ -137,7 +141,8 @@ class Minify_ClosureCompiler
         $this->checkJar(self::$jarFile);
         $server = array(
             self::$javaExecutable,
-            '-jar', escapeshellarg(self::$jarFile)
+            '-jar',
+            escapeshellarg(self::$jarFile)
         );
 
         return $server;
@@ -151,24 +156,13 @@ class Minify_ClosureCompiler
     {
         $args = array();
 
-        $o = array_merge(
-            array(
-                self::OPTION_CHARSET => 'utf-8',
-                self::OPTION_COMPILATION_LEVEL => 'SIMPLE_OPTIMIZATIONS',
-                self::OPTION_WARNING_LEVEL => 'QUIET',
-            ),
+        $options = array_merge(
+            static::$defaultOptions,
             $userOptions
         );
 
-        $charsetOption = $o[self::OPTION_CHARSET];
-        if (preg_match('/^[\\da-zA-Z0-9\\-]+$/', $charsetOption)) {
-            $args[] = "--charset {$charsetOption}";
-        }
-
-        foreach (array(self::OPTION_COMPILATION_LEVEL, self::OPTION_WARNING_LEVEL) as $opt) {
-            if ($o[$opt]) {
-                $args[] = "--{$opt} " . escapeshellarg($o[$opt]);
-            }
+        foreach ($options as $key => $value) {
+            $args[] = "--{$key} " . escapeshellarg($value);
         }
 
         return $args;
diff --git a/tests/LessSourceTest.php b/tests/LessSourceTest.php
index f61045c..a2b5158 100644
--- a/tests/LessSourceTest.php
+++ b/tests/LessSourceTest.php
@@ -24,9 +24,7 @@ class LessSourceTest extends TestCase
         touch($includedLess);
 
         $mtime1 = filemtime($mainLess);
-        var_dump($mtime1);
         $mtime2 = filemtime($includedLess);
-        var_dump($mtime2);
 
         $max = max($mtime1, $mtime2);
 
diff --git a/tests/MinifyClosureCompilerTest.php b/tests/MinifyClosureCompilerTest.php
index ea93eea..84dc1e3 100644
--- a/tests/MinifyClosureCompilerTest.php
+++ b/tests/MinifyClosureCompilerTest.php
@@ -63,7 +63,7 @@ class MinifyClosureCompilerTest extends TestCase
         $src = "function unused() {};";
         $minExpected = '';
         $options = array(
-            Minify_ClosureCompiler::OPTION_COMPILATION_LEVEL => 'ADVANCED_OPTIMIZATIONS'
+            'compilation_level' => 'ADVANCED_OPTIMIZATIONS'
         );
         $minOutput = Minify_ClosureCompiler::minify($src, $options);
         $this->assertSame($minExpected, $minOutput, 'advanced optimizations');
@@ -80,12 +80,36 @@ class MinifyClosureCompilerTest extends TestCase
     {
         $this->assertHasJar();
 
-        $src = file_get_contents(self::$test_files . '/bug-513.js');
+        $src = $this->getDataFile('bug-513.js');
         $minExpected = 'var a=4;';
         $minOutput = Minify_ClosureCompiler::minify($src);
         $this->assertSame($minExpected, $minOutput, 'advanced optimizations');
     }
 
+    /**
+     * Test that language_in parameter has effect.
+     */
+    public function testLanguageOptions()
+    {
+        $this->assertHasJar();
+
+        $src = $this->getDataFile('js/jscomp.polyfill.js');
+        $exp = $this->getDataFile('js/jscomp.polyfill.min.js');
+        $options = array(
+            'language_in' => 'ECMASCRIPT3',
+        );
+
+        $res = Minify_ClosureCompiler::minify($src, $options);
+        $this->assertSame($exp, $res);
+
+        $options = array(
+            'language_in' => 'ECMASCRIPT6',
+        );
+        $exp = $this->getDataFile('js/jscomp.polyfilled.min.js');
+        $res = Minify_ClosureCompiler::minify($src, $options);
+        $this->assertSame($exp, $res);
+    }
+
     protected function assertHasJar()
     {
         $this->assertNotEmpty(Minify_ClosureCompiler::$jarFile);
diff --git a/tests/MinifyNailgunClosureCompilerTest.php b/tests/MinifyNailgunClosureCompilerTest.php
index caeeb70..572ed5f 100644
--- a/tests/MinifyNailgunClosureCompilerTest.php
+++ b/tests/MinifyNailgunClosureCompilerTest.php
@@ -20,7 +20,7 @@ class MinifyNailgunClosureCompilerTest extends TestCase
     /**
      * Test minimisation with the minimum necessary settings
      */
-    public function test2()
+    public function test1()
     {
         $this->assertHasJar();
         $src = "
@@ -40,8 +40,10 @@ class MinifyNailgunClosureCompilerTest extends TestCase
     protected function assertHasJar()
     {
         $this->assertNotEmpty(Minify_ClosureCompiler::$jarFile);
+        $this->assertNotEmpty(Minify_NailgunClosureCompiler::$ngJarFile);
         try {
             $this->assertFileExists(Minify_ClosureCompiler::$jarFile, "Have closure compiler compiler.jar");
+            $this->assertFileExists(Minify_NailgunClosureCompiler::$ngJarFile, "Have nailgun.jar");
         } catch (Exception $e) {
             $this->markTestSkipped($e->getMessage());
         }
diff --git a/tests/ScssSourceTest.php b/tests/ScssSourceTest.php
index a1581ef..c37ca36 100644
--- a/tests/ScssSourceTest.php
+++ b/tests/ScssSourceTest.php
@@ -24,9 +24,7 @@ class ScssSourceTest extends TestCase
         touch($includedLess);
 
         $mtime1 = filemtime($mainLess);
-        var_dump($mtime1);
         $mtime2 = filemtime($includedLess);
-        var_dump($mtime2);
 
         $max = max($mtime1, $mtime2);
 
diff --git a/tests/TestCase.php b/tests/TestCase.php
index a871461..f763412 100644
--- a/tests/TestCase.php
+++ b/tests/TestCase.php
@@ -47,4 +47,23 @@ class TestCase extends PHPUnit_Framework_TestCase
         $this->assertSame($data, $displayed, "$id display");
         $this->assertEquals($data, $cache->fetch($id), "$id fetch");
     }
+
+    /**
+     * Read data file, assert that it exists and is not empty.
+     * As a side effect calls trim() to fight against different Editors that insert or strip final newline.
+     *
+     * @param string $filename
+     * @return string
+     */
+    protected function getDataFile($filename)
+    {
+        $path = self::$test_files . '/' . $filename;
+        $this->assertFileExists($path);
+        $contents = file_get_contents($path);
+        $this->assertNotEmpty($contents);
+        $contents = trim($contents);
+        $this->assertNotEmpty($contents);
+
+        return $contents;
+    }
 }
\ No newline at end of file
diff --git a/tests/_test_files/js/jscomp.polyfill.js b/tests/_test_files/js/jscomp.polyfill.js
new file mode 100644
index 0000000..f1da735
--- /dev/null
+++ b/tests/_test_files/js/jscomp.polyfill.js
@@ -0,0 +1,7 @@
+(function() {
+    /**
+     * @type {string}
+     */
+    var $array = jQuery.find('#div');
+    print($array.find('a'));
+})();
\ No newline at end of file
diff --git a/tests/_test_files/js/jscomp.polyfill.min.js b/tests/_test_files/js/jscomp.polyfill.min.js
new file mode 100644
index 0000000..6146151
--- /dev/null
+++ b/tests/_test_files/js/jscomp.polyfill.min.js
@@ -0,0 +1 @@
+(function(){var a=jQuery.find("#div");print(a.find("a"))})();
\ No newline at end of file
diff --git a/tests/_test_files/js/jscomp.polyfilled.min.js b/tests/_test_files/js/jscomp.polyfilled.min.js
new file mode 100644
index 0000000..faa4d04
--- /dev/null
+++ b/tests/_test_files/js/jscomp.polyfilled.min.js
@@ -0,0 +1,3 @@
+var $jscomp={scope:{},findInternal:function(a,c,b){a instanceof String&&(a=String(a));for(var d=a.length,e=0;e<d;e++){var f=a[e];if(c.call(b,f,e,a))return{i:e,v:f}}return{i:-1,v:void 0}}};$jscomp.defineProperty="function"==typeof Object.defineProperties?Object.defineProperty:function(a,c,b){if(b.get||b.set)throw new TypeError("ES3 does not support getters and setters.");a!=Array.prototype&&a!=Object.prototype&&(a[c]=b.value)};
+$jscomp.getGlobal=function(a){return"undefined"!=typeof window&&window===a?a:"undefined"!=typeof global&&null!=global?global:a};$jscomp.global=$jscomp.getGlobal(this);$jscomp.polyfill=function(a,c,b,d){if(c){b=$jscomp.global;a=a.split(".");for(d=0;d<a.length-1;d++){var e=a[d];e in b||(b[e]={});b=b[e]}a=a[a.length-1];d=b[a];c=c(d);c!=d&&null!=c&&$jscomp.defineProperty(b,a,{configurable:!0,writable:!0,value:c})}};
+$jscomp.polyfill("Array.prototype.find",function(a){return a?a:function(a,b){return $jscomp.findInternal(this,a,b).v}},"es6-impl","es3");(function(){var a=jQuery.find("#div");print(a.find("a"))})();
\ No newline at end of file