Special-case top-level final JS fields.

lib/runtime/dart/js.js

 dart_library.library('dart/js', null, /* Imports */[
   "dart/_runtime",
   'dart/core',
   'dart/collection',
   'dart/_js_helper'
 ], /* Lazy imports */[
 ], function(exports, dart, core, collection, _js_helper) {
   'use strict';
   let dartx = dart.dartx;
   dart.defineLazyProperties(exports, {
@@ skipping 317 matching lines @@
     constructors: () => ({_DartObject: [_DartObject, [dart.dynamic]]})
   });
   function _convertToJS(o) {
     if (o == null || typeof o == 'string' || typeof o == 'number' || typeof o == 'boolean' || dart.notNull(_isBrowserType(o))) {
       return o;
     } else if (dart.is(o, core.DateTime)) {
       return _js_helper.Primitives.lazyAsJsDate(o);
     } else if (dart.is(o, JsObject)) {
       return dart.dload(o, _jsObject);
     } else if (dart.is(o, core.Function)) {
-      return _putIfAbsent(exports._jsProxies, o, _wrapDartFunction);
+      return _putIfAbsent(_jsProxies, o, _wrapDartFunction);
     } else {
-      return _putIfAbsent(exports._jsProxies, o, dart.fn(o => new _DartObject(o), _DartObject, [dart.dynamic]));
+      return _putIfAbsent(_jsProxies, o, dart.fn(o => new _DartObject(o), _DartObject, [dart.dynamic]));
     }
   }
   dart.fn(_convertToJS);
   function _wrapDartFunction(f) {
     let wrapper = function() {
       let args = Array.prototype.map.call(arguments, _convertToDart);
       return _convertToJS(f(...args));
     };
-    dart.dsetindex(exports._dartProxies, wrapper, f);
+    dart.dsetindex(_dartProxies, wrapper, f);
     return wrapper;
   }
   dart.fn(_wrapDartFunction);
   function _convertToDart(o) {
     if (o == null || typeof o == "string" || typeof o == "number" || typeof o == "boolean" || dart.notNull(_isBrowserType(o))) {
       return o;
     } else if (o instanceof Date) {
       let ms = o.getTime();
       return new core.DateTime.fromMillisecondsSinceEpoch(dart.asInt(ms));
     } else if (dart.is(o, _DartObject) && dart.jsobject != dart.realRuntimeType(o)) {
       return dart.dload(o, _dartObj);
     } else {
-      return _putIfAbsent(exports._dartProxies, o, _wrapToDart);
+      return _putIfAbsent(_dartProxies, o, _wrapToDart);
     }
   }
   dart.fn(_convertToDart, core.Object, [dart.dynamic]);
   function _wrapToDart(o) {
     if (typeof o == "function") {
       return new JsFunction._fromJs(o);
     }
     if (o instanceof Array) {
       return new JsArray._fromJs(o);
     }
     return new JsObject._fromJs(o);
   }
   dart.fn(_wrapToDart, JsObject, [dart.dynamic]);
-  dart.defineLazyProperties(exports, {
-    get _dartProxies() {
-      return new WeakMap();
-    },
-    get _jsProxies() {
-      return new WeakMap();
-    }
-  });
+  const _dartProxies = new WeakMap();
+  const _jsProxies = new WeakMap();
   function _putIfAbsent(weakMap, o, getValue) {
     let value = weakMap.get(o);
     if (value == null) {
       value = dart.dcall(getValue, o);
       weakMap.set(o, value);
     }
     return value;
   }
   dart.fn(_putIfAbsent, core.Object, [dart.dynamic, dart.dynamic, dart.functionType(dart.dynamic, [dart.dynamic])]);
   function allowInterop(f) {
@@ skipping 27 matching lines @@
   });
   let __CastType0 = __CastType0$();
   // Exports:
   exports.JsObject = JsObject;
   exports.JsFunction = JsFunction;
   exports.JsArray$ = JsArray$;
   exports.JsArray = JsArray;
   exports.allowInterop = allowInterop;
   exports.allowInteropCaptureThis = allowInteropCaptureThis;
 });