Accept 'null' as document.contentScript.

pkg/compiler/lib/src/js_emitter/main_call_stub_generator.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart2js.js_emitter;
 
 class MainCallStubGenerator {
   final Compiler compiler;
   final JavaScriptBackend backend;
   final CodeEmitterTask emitterTask;
@@ skipping 34 matching lines @@
     // This code finds the currently executing script by listening to the
     // onload event of all script tags and getting the first script which
     // finishes. Since onload is called immediately after execution this should
     // not substantially change execution order.
     return js.statement('''
       (function (callback) {
         if (typeof document === "undefined") {
           callback(null);
           return;
         }
-        if (document.currentScript) {
+        // When running as a content-script of a chrome-extension the
+        // 'currentScript' is `null` (but not undefined).
+        if (typeof document.currentScript != 'undefined') {
           callback(document.currentScript);
           return;
         }
 
         var scripts = document.scripts;
         function onLoad(event) {
           for (var i = 0; i < scripts.length; ++i) {
             scripts[i].removeEventListener("load", onLoad, false);
           }
           callback(event.target);
         }
         for (var i = 0; i < scripts.length; ++i) {
           scripts[i].addEventListener("load", onLoad, false);
         }
       })(function(currentScript) {
         #currentScript = currentScript;
 
         if (typeof dartMainRunner === "function") {
           dartMainRunner(#mainCallClosure, []);
         } else {
           #mainCallClosure([]);
         }
       })''',
       {'currentScript': currentScriptAccess,
        'mainCallClosure': mainCallClosure});
   }
 }