Fix custom formatter setup, take 2

pkg/dev_compiler/lib/js/legacy/dart_library.js

 // Copyright (c) 2015, 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.
 
 /* This file defines the module loader for the dart runtime.
 */
 var dart_library;
 if (!dart_library) {
 dart_library =
   typeof module != "undefined" && module.exports || {};
@@ skipping 135 matching lines @@
       console.warn('Already loaded ' + name);
       return result;
     }
     result = new LibraryLoader(name, defaultValue, imports, loader);
     libraries.set(name, result);
     return result;
   }
   dart_library.library = library;
 
   function import_(libraryName) {
-    bootstrap();
     let loader = libraries.get(libraryName);
     // TODO(vsm): A user might call this directly from JS (as we do in tests).
     // We may want a different error type.
     if (!loader) throwLibraryError('Library not found: ' + libraryName);
     return loader.load();
   }
   dart_library.import = import_;
 
   var _currentIsolate = false;
 
   function start(moduleName, libraryName) {
     if (libraryName == null) libraryName = moduleName;
     let library = import_(moduleName)[libraryName];
     let dart_sdk = import_('dart_sdk');
 
-    // This import is only needed for chrome debugging. We should provide an
-    // option to compile without it.
-    dart_sdk._debugger.registerDevtoolsFormatter();
+    if (!_currentIsolate) {
+      // This import is only needed for chrome debugging. We should provide an
+      // option to compile without it.
+      dart_sdk._debugger.registerDevtoolsFormatter();
 
-    if (!_currentIsolate) {
       // Create isolate.
       _currentIsolate = true;
       dart_sdk._isolate_helper.startRootIsolate(() => {}, []);
     }
 
     library.main();
   }
   dart_library.start = start;
 
-  let _bootstrapped = false;
-  function bootstrap() {
-    if (_bootstrapped) return;
-    _bootstrapped = true;
-  }
-
 })(dart_library);
 }