allow JS builtin to be typed as needed

lib/runtime/dart/_isolate_helper.js

 dart_library.library('dart/_isolate_helper', null, /* Imports */[
   'dart/_runtime',
   'dart/core',
   'dart/_interceptors',
   'dart/_js_helper',
   'dart/isolate',
   'dart/_foreign_helper',
   'dart/collection',
   'dart/async'
 ], /* Lazy imports */[
@@ skipping 93 matching lines @@
       return x;
     }
     serializeMap(x) {
       let serializeTearOff = dart.bind(this, 'serialize');
       return ['map', x.keys[dartx.map](dart.as(serializeTearOff, __CastType0))[dartx.toList](), x.values[dartx.map](dart.as(serializeTearOff, dart.functionType(dart.dynamic, [dart.dynamic])))[dartx.toList]()];
     }
     serializeJSObject(x) {
       if (!!x.constructor && x.constructor !== Object) {
         this.unsupported(x, "Only plain JS Objects are supported:");
       }
-      let keys = dart.as(Object.keys(x), core.List);
+      let keys = Object.keys(x);
       let values = [];
       values[dartx.length] = keys[dartx.length];
       for (let i = 0; dart.notNull(i) < dart.notNull(keys[dartx.length]); i = dart.notNull(i) + 1) {
         values[dartx.set](i, this.serialize(x[keys[dartx.get](i)]));
       }
       return ['js-object', keys, values];
     }
     serializeWorkerSendPort(x) {
       if (dart.notNull(this[_serializeSendPorts])) {
         return ['sendport', x[_workerId], x[_isolateId], x[_receivePortId]];
@@ skipping 14 matching lines @@
       let name = IsolateNatives._getJSFunctionName(x);
       if (name == null) {
         this.unsupported(x, "Closures can't be transmitted:");
       }
       return ['function', name];
     }
     serializeDartObject(x) {
       let classExtractor = _foreign_helper.JS_EMBEDDED_GLOBAL('', _js_embedded_names.CLASS_ID_EXTRACTOR);
       let fieldsExtractor = _foreign_helper.JS_EMBEDDED_GLOBAL('', _js_embedded_names.CLASS_FIELDS_EXTRACTOR);
       let classId = classExtractor(x);
-      let fields = dart.as(fieldsExtractor(x), core.List);
+      let fields = fieldsExtractor(x);
       return ['dart', classId, this.serializeArrayInPlace(dart.as(fields, _interceptors.JSArray))];
     }
   }
   dart.setSignature(_Serializer, {
     constructors: () => ({_Serializer: [_Serializer, [], {serializeSendPorts: dart.dynamic}]}),
     methods: () => ({
       serialize: [dart.dynamic, [dart.dynamic]],
       unsupported: [dart.void, [dart.dynamic], [core.String]],
       makeRef: [dart.dynamic, [core.int]],
       isPrimitive: [core.bool, [dart.dynamic]],
@@ skipping 269 matching lines @@
         dart.dcall(entry, args, null);
       }));
     } else {
       rootContext.eval(dart.as(entry, core.Function));
     }
     exports._globalState.topEventLoop.run();
   }
   dart.fn(startRootIsolate, dart.void, [dart.dynamic, dart.dynamic]);
   dart.copyProperties(exports, {
     get _globalState() {
-      return dart.as(dart.globalState, _Manager);
+      return dart.globalState;
     },
     set _globalState(val) {
       dart.globalState = val;
     }
   });
   const _nativeDetectEnvironment = Symbol('_nativeDetectEnvironment');
   const _nativeInitWorkerMessageHandler = Symbol('_nativeInitWorkerMessageHandler');
   class _Manager extends core.Object {
     get useWorkers() {
       return this.supportsWorkers;
@@ skipping 465 matching lines @@
         return String(currentScript.src);
       }
       if (dart.notNull(_js_helper.Primitives.isD8)) return IsolateNatives.computeThisScriptD8();
       if (dart.notNull(_js_helper.Primitives.isJsshell)) return IsolateNatives.computeThisScriptJsshell();
       if (exports._globalState != null && dart.notNull(exports._globalState.isWorker)) {
         return IsolateNatives.computeThisScriptFromTrace();
       }
       return null;
     }
     static computeThisScriptJsshell() {
-      return dart.as(thisFilename(), core.String);
+      return thisFilename();
     }
     static computeThisScriptD8() {
       return IsolateNatives.computeThisScriptFromTrace();
     }
     static computeThisScriptFromTrace() {
       let stack = new Error().stack;
       if (stack == null) {
         stack = (function() {
           try {
             throw new Error();
@@ skipping 100 matching lines @@
       }
     }
     static _consoleLog(msg) {
       self.console.log(msg);
     }
     static _getJSFunctionFromName(functionName) {
       let globalFunctionsContainer = _foreign_helper.JS_EMBEDDED_GLOBAL("", _js_embedded_names.GLOBAL_FUNCTIONS);
       return globalFunctionsContainer[functionName]();
     }
     static _getJSFunctionName(f) {
-      return dart.as(f.$name, core.String);
+      return f.$name;
     }
     static _allocate(ctor) {
       return new ctor();
     }
     static spawnFunction(topLevelFunction, message, startPaused) {
       IsolateNatives.enableSpawnWorker = true;
       let name = IsolateNatives._getJSFunctionName(topLevelFunction);
       if (name == null) {
         dart.throw(new core.UnsupportedError("only top-level functions can be spawned."));
       }
@@ skipping 94 matching lines @@
       worker.onmessage = processWorkerMessageTrampoline;
       let o = exports._globalState;
       let workerId = o.nextManagerId;
       o.nextManagerId = dart.notNull(workerId) + 1;
       IsolateNatives.workerIds.set(worker, workerId);
       exports._globalState.managers.set(workerId, worker);
       worker.postMessage(_serializeMessage(dart.map({command: 'start', id: workerId, replyTo: _serializeMessage(replyPort), args: args, msg: _serializeMessage(message), isSpawnUri: isSpawnUri, startPaused: startPaused, functionName: functionName})));
     }
     static workerOnError(event, uri, onError) {
       event.preventDefault();
-      let message = dart.as(event.message, core.String);
+      let message = event.message;
       if (message == null) {
         message = `Error spawning worker for ${uri}`;
       } else {
         message = `Error spawning worker for ${uri} (${message})`;
       }
       onError(message);
       return true;
     }
   }
   dart.setSignature(IsolateNatives, {
@@ skipping 336 matching lines @@
   exports.leaveJsAsync = leaveJsAsync;
   exports.isWorker = isWorker;
   exports.startRootIsolate = startRootIsolate;
   exports.IsolateNatives = IsolateNatives;
   exports.RawReceivePortImpl = RawReceivePortImpl;
   exports.ReceivePortImpl = ReceivePortImpl;
   exports.TimerImpl = TimerImpl;
   exports.hasTimer = hasTimer;
   exports.CapabilityImpl = CapabilityImpl;
 });