allow JS builtin to be typed as needed

lib/runtime/dart/_isolate_helper.js

Index: lib/runtime/dart/_isolate_helper.js
diff --git a/lib/runtime/dart/_isolate_helper.js b/lib/runtime/dart/_isolate_helper.js
index 9135e27040ed0c74a71f3312103f022835785e3b..62fe8e8da208ec5bee5449cb7e4c124599de6a8e 100644
--- a/lib/runtime/dart/_isolate_helper.js
+++ b/lib/runtime/dart/_isolate_helper.js
@@ -111,7 +111,7 @@ dart_library.library('dart/_isolate_helper', null, /* Imports */[
       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) {
@@ -146,7 +146,7 @@ dart_library.library('dart/_isolate_helper', null, /* Imports */[
       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))];
     }
   }
@@ -436,7 +436,7 @@ dart_library.library('dart/_isolate_helper', null, /* Imports */[
   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;
@@ -922,7 +922,7 @@ dart_library.library('dart/_isolate_helper', null, /* Imports */[
       return null;
     }
     static computeThisScriptJsshell() {
-      return dart.as(thisFilename(), core.String);
+      return thisFilename();
     }
     static computeThisScriptD8() {
       return IsolateNatives.computeThisScriptFromTrace();
@@ -1043,7 +1043,7 @@ dart_library.library('dart/_isolate_helper', null, /* Imports */[
       return globalFunctionsContainer[functionName]();
     }
     static _getJSFunctionName(f) {
-      return dart.as(f.$name, core.String);
+      return f.$name;
     }
     static _allocate(ctor) {
       return new ctor();
@@ -1158,7 +1158,7 @@ dart_library.library('dart/_isolate_helper', null, /* Imports */[
     }
     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 {