Special-case top-level final JS fields.

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 898 matching lines @@
         this.isolate.delayedEvents[dartx.add](this);
         return;
       }
       this.isolate.eval(this.fn);
     }
   }
   dart.setSignature(_IsolateEvent, {
     constructors: () => ({_IsolateEvent: [_IsolateEvent, [_IsolateContext, core.Function, core.String]]}),
     methods: () => ({process: [dart.void, []]})
   });
-  dart.defineLazyProperties(exports, {
-    get _global() {
-      return typeof global == 'undefined' ? self : global;
-    }
-  });
+  const _global = typeof global == 'undefined' ? self : global;
   class _MainManagerStub extends core.Object {
     postMessage(msg) {
-      exports._global.postMessage(msg);
+      _global.postMessage(msg);
     }
   }
   dart.setSignature(_MainManagerStub, {
     methods: () => ({postMessage: [dart.void, [dart.dynamic]]})
   });
   const _SPAWNED_SIGNAL = "spawned";
   const _SPAWN_FAILED_SIGNAL = "spawn failed";
   dart.copyProperties(exports, {
     get globalWindow() {
-      return exports._global.window;
+      return _global.window;
     },
     get globalWorker() {
-      return exports._global.Worker;
+      return _global.Worker;
     },
     get globalPostMessageDefined() {
-      return !!exports._global.postMessage;
+      return !!_global.postMessage;
     }
   });
   const _MainFunction = dart.typedef('_MainFunction', () => dart.functionType(dart.dynamic, []));
   const _MainFunctionArgs = dart.typedef('_MainFunctionArgs', () => dart.functionType(dart.dynamic, [dart.dynamic]));
   const _MainFunctionArgsMessage = dart.typedef('_MainFunctionArgsMessage', () => dart.functionType(dart.dynamic, [dart.dynamic, dart.dynamic]));
   class IsolateNatives extends core.Object {
     static computeThisScript() {
       let currentScript = document.currentScript;
       if (currentScript != null) {
         return String(currentScript.src);
@@ skipping 607 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;
 });