Use const for const/final top-levels, types, symbols.

lib/runtime/dart/isolate.js

 dart_library.library('dart/isolate', null, /* Imports */[
   "dart/_runtime",
   'dart/core',
   'dart/async'
 ], /* Lazy imports */[
   'dart/_isolate_helper'
 ], function(exports, dart, core, async, _isolate_helper) {
   'use strict';
   let dartx = dart.dartx;
   class Capability extends core.Object {
     static new() {
       return new _isolate_helper.CapabilityImpl();
     }
   }
   dart.setSignature(Capability, {
     constructors: () => ({new: [Capability, []]})
   });
   class IsolateSpawnException extends core.Object {
     IsolateSpawnException(message) {
       this.message = message;
     }
     toString() {
       return `IsolateSpawnException: ${this.message}`;
     }
   }
   IsolateSpawnException[dart.implements] = () => [core.Exception];
   dart.setSignature(IsolateSpawnException, {
     constructors: () => ({IsolateSpawnException: [IsolateSpawnException, [core.String]]})
   });
-  let _pause = Symbol('_pause');
+  const _pause = Symbol('_pause');
   class Isolate extends core.Object {
     Isolate(controlPort, opts) {
       let pauseCapability = opts && 'pauseCapability' in opts ? opts.pauseCapability : null;
       let terminateCapability = opts && 'terminateCapability' in opts ? opts.terminateCapability : null;
       this.controlPort = controlPort;
       this.pauseCapability = pauseCapability;
       this.terminateCapability = terminateCapability;
     }
     static get current() {
       return Isolate._currentIsolateCache;
@@ skipping 179 matching lines @@
       this.stackTrace = stackTrace;
     }
     toString() {
       return 'IsolateUnhandledException: exception while handling message: ' + `${this.message} \n  ` + `${dart.toString(this.source)[dartx.replaceAll]("\n", "\n  ")}\n` + 'original stack trace:\n  ' + `${dart.toString(this.stackTrace)[dartx.replaceAll]("\n", "\n  ")}`;
     }
   }
   _IsolateUnhandledException[dart.implements] = () => [core.Exception];
   dart.setSignature(_IsolateUnhandledException, {
     constructors: () => ({_IsolateUnhandledException: [_IsolateUnhandledException, [dart.dynamic, dart.dynamic, core.StackTrace]]})
   });
-  let _description = Symbol('_description');
+  const _description = Symbol('_description');
   class RemoteError extends core.Object {
     RemoteError(description, stackDescription) {
       this[_description] = description;
       this.stackTrace = new _RemoteStackTrace(stackDescription);
     }
     toString() {
       return this[_description];
     }
   }
   RemoteError[dart.implements] = () => [core.Error];
   dart.setSignature(RemoteError, {
     constructors: () => ({RemoteError: [RemoteError, [core.String, core.String]]})
   });
-  let _trace = Symbol('_trace');
+  const _trace = Symbol('_trace');
   class _RemoteStackTrace extends core.Object {
     _RemoteStackTrace(trace) {
       this[_trace] = trace;
     }
     toString() {
       return this[_trace];
     }
   }
   _RemoteStackTrace[dart.implements] = () => [core.StackTrace];
   dart.setSignature(_RemoteStackTrace, {
     constructors: () => ({_RemoteStackTrace: [_RemoteStackTrace, [core.String]]})
   });
   // Exports:
   exports.Capability = Capability;
   exports.IsolateSpawnException = IsolateSpawnException;
   exports.Isolate = Isolate;
   exports.SendPort = SendPort;
   exports.ReceivePort = ReceivePort;
   exports.RawReceivePort = RawReceivePort;
   exports.RemoteError = RemoteError;
 });