allow JS builtin to be typed as needed

lib/runtime/dart/convert.js

 dart_library.library('dart/convert', null, /* Imports */[
   'dart/_runtime',
   'dart/core',
   'dart/async',
   'dart/typed_data',
   'dart/_internal',
   'dart/collection'
 ], /* Lazy imports */[
 ], function(exports, dart, core, async, typed_data, _internal, collection) {
   'use strict';
@@ skipping 2639 matching lines @@
       }
     }
     toString() {
       return collection.Maps.mapToString(this);
     }
     get [_isUpgraded]() {
       return this[_processed] == null;
     }
     get [_upgradedMap]() {
       dart.assert(this[_isUpgraded]);
-      return dart.as(this[_data], core.Map);
+      return this[_data];
     }
     [_computeKeys]() {
       dart.assert(!dart.notNull(this[_isUpgraded]));
       let keys = dart.as(this[_data], core.List);
       if (keys == null) {
         keys = this[_data] = _JsonMap._getPropertyNames(this[_original]);
       }
-      return dart.as(keys, core.List$(core.String));
+      return keys;
     }
     [_upgrade]() {
       if (dart.notNull(this[_isUpgraded])) return this[_upgradedMap];
       let result = dart.map();
       let keys = this[_computeKeys]();
       for (let i = 0; dart.notNull(i) < dart.notNull(keys[dartx.length]); i = dart.notNull(i) + 1) {
         let key = keys[dartx.get](i);
         result.set(key, this.get(key));
       }
       if (dart.notNull(keys[dartx.isEmpty])) {
@@ skipping 14 matching lines @@
     static _hasProperty(object, key) {
       return Object.prototype.hasOwnProperty.call(object, key);
     }
     static _getProperty(object, key) {
       return object[key];
     }
     static _setProperty(object, key, value) {
       return object[key] = value;
     }
     static _getPropertyNames(object) {
-      return dart.as(Object.keys(object), core.List);
+      return Object.keys(object);
     }
     static _isUnprocessed(object) {
       return typeof object == "undefined";
     }
     static _newJavaScriptObject() {
       return Object.create(null);
     }
   }
   _JsonMap[dart.implements] = () => [collection.LinkedHashMap];
   dart.setSignature(_JsonMap, {
@@ skipping 98 matching lines @@
   exports.LineSplitter = LineSplitter;
   exports.StringConversionSink = StringConversionSink;
   exports.ClosableStringSink = ClosableStringSink;
   exports.UNICODE_REPLACEMENT_CHARACTER_RUNE = UNICODE_REPLACEMENT_CHARACTER_RUNE;
   exports.UNICODE_BOM_CHARACTER_RUNE = UNICODE_BOM_CHARACTER_RUNE;
   exports.Utf8Codec = Utf8Codec;
   exports.UTF8 = UTF8;
   exports.Utf8Encoder = Utf8Encoder;
   exports.Utf8Decoder = Utf8Decoder;
 });