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

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, {
     get context() {
       return _wrapToDart(dart.global);
     }
   });
-  let _jsObject = Symbol('_jsObject');
+  const _jsObject = Symbol('_jsObject');
   class JsObject extends core.Object {
     _fromJs(jsObject) {
       this[_jsObject] = jsObject;
       dart.assert(this[_jsObject] != null);
     }
     static new(constructor, arguments$) {
       if (arguments$ === void 0)
         arguments$ = null;
       let ctor = constructor[_jsObject];
       if (arguments$ == null) {
@@ skipping 133 matching lines @@
     }
   }
   dart.defineNamedConstructor(JsFunction, '_fromJs');
   dart.setSignature(JsFunction, {
     constructors: () => ({
       withThis: [JsFunction, [core.Function]],
       _fromJs: [JsFunction, [dart.dynamic]]
     }),
     methods: () => ({apply: [dart.dynamic, [core.List], {thisArg: dart.dynamic}]})
   });
-  let _checkIndex = Symbol('_checkIndex');
-  let _checkInsertIndex = Symbol('_checkInsertIndex');
-  let JsArray$ = dart.generic(function(E) {
+  const _checkIndex = Symbol('_checkIndex');
+  const _checkInsertIndex = Symbol('_checkInsertIndex');
+  const JsArray$ = dart.generic(function(E) {
     class JsArray extends dart.mixin(JsObject, collection.ListMixin$(E)) {
       JsArray() {
         super._fromJs([]);
       }
       from(other) {
         super._fromJs((() => {
           let _ = [];
           _[dartx.addAll](other[dartx.map](dart.as(_convertToJS, __CastType0$(E))));
           return _;
         })());
@@ skipping 129 matching lines @@
       'length',
       'length'
     ]);
     return JsArray;
   });
   let JsArray = JsArray$();
   function _isBrowserType(o) {
     return o instanceof Blob || o instanceof Event || window.KeyRange && o instanceof KeyRange || o instanceof ImageData || o instanceof Node || window.TypedData && o instanceof TypedData || o instanceof Window;
   }
   dart.fn(_isBrowserType, core.bool, [dart.dynamic]);
-  let _dartObj = Symbol('_dartObj');
+  const _dartObj = Symbol('_dartObj');
   class _DartObject extends core.Object {
     _DartObject(dartObj) {
       this[_dartObj] = dartObj;
     }
   }
   dart.setSignature(_DartObject, {
     constructors: () => ({_DartObject: [_DartObject, [dart.dynamic]]})
   });
   function _convertToJS(o) {
     if (o == null || typeof o == 'string' || typeof o == 'number' || typeof o == 'boolean' || dart.notNull(_isBrowserType(o))) {
@@ skipping 76 matching lines @@
         for (let arg of arguments) {
           args.push(arg);
         }
         return f(...args);
       }, core.Function);
       exports._interopCaptureThisExpando.set(f, ret);
     }
     return ret;
   }
   dart.fn(allowInteropCaptureThis, core.Function, [core.Function]);
-  let __CastType0$ = dart.generic(function(E) {
-    let __CastType0 = dart.typedef('__CastType0', () => dart.functionType(dart.dynamic, [E]));
+  const __CastType0$ = dart.generic(function(E) {
+    const __CastType0 = dart.typedef('__CastType0', () => dart.functionType(dart.dynamic, [E]));
     return __CastType0;
   });
   let __CastType0 = __CastType0$();
   // Exports:
   exports.JsObject = JsObject;
   exports.JsFunction = JsFunction;
   exports.JsArray$ = JsArray$;
   exports.JsArray = JsArray;
   exports.allowInterop = allowInterop;
   exports.allowInteropCaptureThis = allowInteropCaptureThis;
 });