fixes #168, dart:js implementation with a test

lib/runtime/dart/_js_helper.js

 dart_library.library('dart/_js_helper', null, /* Imports */[
   "dart_runtime/dart",
   'dart/core',
   'dart/collection',
   'dart/_interceptors',
   'dart/_foreign_helper'
 ], /* Lazy imports */[
 ], function(exports, dart, core, collection, _interceptors, _foreign_helper) {
   'use strict';
   let dartx = dart.dartx;
@@ skipping 1015 matching lines @@
       let value = getIndex(keyValuePairs, (() => {
         let x = index;
         index = dart.notNull(x) + 1;
         return x;
       })());
       result.set(key, value);
     }
     return result;
   }
   dart.fn(fillLiteralMap, dart.dynamic, [dart.dynamic, core.Map]);
-  function convertDartClosureToJS(closure, arity) {
-    return closure;
-  }
-  dart.fn(convertDartClosureToJS, dart.dynamic, [dart.dynamic, core.int]);
   function jsHasOwnProperty(jsObject, property) {
     return jsObject.hasOwnProperty(property);
   }
   dart.fn(jsHasOwnProperty, core.bool, [dart.dynamic, core.String]);
   function jsPropertyAccess(jsObject, property) {
     return jsObject[property];
   }
   dart.fn(jsPropertyAccess, dart.dynamic, [dart.dynamic, core.String]);
   function getFallThroughError() {
     return new FallThroughErrorImplementation();
@@ skipping 130 matching lines @@
   exports.checkBool = checkBool;
   exports.checkString = checkString;
   exports.throwRuntimeError = throwRuntimeError;
   exports.throwAbstractClassInstantiationError = throwAbstractClassInstantiationError;
   exports.NullError = NullError;
   exports.JsNoSuchMethodError = JsNoSuchMethodError;
   exports.UnknownJsTypeError = UnknownJsTypeError;
   exports.getTraceFromException = getTraceFromException;
   exports.objectHashCode = objectHashCode;
   exports.fillLiteralMap = fillLiteralMap;
-  exports.convertDartClosureToJS = convertDartClosureToJS;
   exports.jsHasOwnProperty = jsHasOwnProperty;
   exports.jsPropertyAccess = jsPropertyAccess;
   exports.getFallThroughError = getFallThroughError;
   exports.Creates = Creates;
   exports.Returns = Returns;
   exports.JSName = JSName;
   exports.JavaScriptIndexingBehavior = JavaScriptIndexingBehavior;
   exports.TypeErrorImplementation = TypeErrorImplementation;
   exports.CastErrorImplementation = CastErrorImplementation;
   exports.FallThroughErrorImplementation = FallThroughErrorImplementation;
   exports.RuntimeError = RuntimeError;
   exports.random64 = random64;
   exports.jsonEncodeNative = jsonEncodeNative;
 });