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

lib/runtime/dart/_interceptors.js

 dart_library.library('dart/_interceptors', null, /* Imports */[
   "dart/_runtime",
   'dart/core',
   'dart/_internal',
   'dart/collection',
   'dart/math'
 ], /* Lazy imports */[
   'dart/_js_helper'
 ], function(exports, dart, core, _internal, collection, math, _js_helper) {
   'use strict';
   let dartx = dart.dartx;
-  let JSArray$ = dart.generic(function(E) {
+  const JSArray$ = dart.generic(function(E) {
     dart.defineExtensionNames([
       'checkGrowable',
       'add',
       'removeAt',
       'insert',
       'insertAll',
       'setAll',
       'removeLast',
       'remove',
       'removeWhere',
@@ skipping 426 matching lines @@
         [dartx.asMap]: [core.Map$(core.int, E), []]
       }),
       statics: () => ({markFixedList: [core.List, [core.List]]}),
       names: ['markFixedList']
     });
     JSArray[dart.metadata] = () => [dart.const(new _js_helper.JsPeerInterface({name: 'Array'}))];
     return JSArray;
   });
   let JSArray = JSArray$();
   dart.registerExtension(dart.global.Array, JSArray);
-  let JSMutableArray$ = dart.generic(function(E) {
+  const JSMutableArray$ = dart.generic(function(E) {
     class JSMutableArray extends JSArray$(E) {
       JSMutableArray() {
         super.JSArray();
       }
     }
     JSMutableArray[dart.implements] = () => [JSMutableIndexable];
     return JSMutableArray;
   });
   let JSMutableArray = JSMutableArray$();
-  let JSFixedArray$ = dart.generic(function(E) {
+  const JSFixedArray$ = dart.generic(function(E) {
     class JSFixedArray extends JSMutableArray$(E) {}
     return JSFixedArray;
   });
   let JSFixedArray = JSFixedArray$();
-  let JSExtendableArray$ = dart.generic(function(E) {
+  const JSExtendableArray$ = dart.generic(function(E) {
     class JSExtendableArray extends JSMutableArray$(E) {}
     return JSExtendableArray;
   });
   let JSExtendableArray = JSExtendableArray$();
   class Interceptor extends core.Object {
     Interceptor() {
     }
   }
   dart.setSignature(Interceptor, {
     constructors: () => ({Interceptor: [Interceptor, []]})
   });
-  let _isInt32 = Symbol('_isInt32');
-  let _tdivSlow = Symbol('_tdivSlow');
-  let _shlPositive = Symbol('_shlPositive');
-  let _shrOtherPositive = Symbol('_shrOtherPositive');
-  let _shrBothPositive = Symbol('_shrBothPositive');
+  const _isInt32 = Symbol('_isInt32');
+  const _tdivSlow = Symbol('_tdivSlow');
+  const _shlPositive = Symbol('_shlPositive');
+  const _shrOtherPositive = Symbol('_shrOtherPositive');
+  const _shrBothPositive = Symbol('_shrBothPositive');
   dart.defineExtensionNames([
     'compareTo',
     'isNegative',
     'isNaN',
     'isInfinite',
     'isFinite',
     'remainder',
     'abs',
     'sign',
     'toInt',
@@ skipping 404 matching lines @@
       _shrs: [core.int, [core.int, core.int]],
       _ors: [core.int, [core.int, core.int]],
       _spread: [core.int, [core.int]]
     }),
     names: ['_handleIEtoString', '_bitCount', '_shru', '_shrs', '_ors', '_spread']
   });
   JSNumber[dart.metadata] = () => [dart.const(new _js_helper.JsPeerInterface({name: 'Number'}))];
   dart.registerExtension(dart.global.Number, JSNumber);
   JSNumber._MIN_INT32 = -2147483648;
   JSNumber._MAX_INT32 = 2147483647;
-  let _defaultSplit = Symbol('_defaultSplit');
+  const _defaultSplit = Symbol('_defaultSplit');
   dart.defineExtensionNames([
     'codeUnitAt',
     'allMatches',
     'matchAsPrefix',
     '+',
     'endsWith',
     'replaceAll',
     'replaceAllMapped',
     'splitMapJoin',
     'replaceFirst',
@@ skipping 472 matching lines @@
     }),
     statics: () => ({
       _isWhitespace: [core.bool, [core.int]],
       _skipLeadingWhitespace: [core.int, [core.String, core.int]],
       _skipTrailingWhitespace: [core.int, [core.String, core.int]]
     }),
     names: ['_isWhitespace', '_skipLeadingWhitespace', '_skipTrailingWhitespace']
   });
   JSString[dart.metadata] = () => [dart.const(new _js_helper.JsPeerInterface({name: 'String'}))];
   dart.registerExtension(dart.global.String, JSString);
-  let _string = Symbol('_string');
+  const _string = Symbol('_string');
   class _CodeUnits extends _internal.UnmodifiableListBase$(core.int) {
     _CodeUnits(string) {
       this[_string] = string;
     }
     get length() {
       return this[_string][dartx.length];
     }
     get(i) {
       return this[_string][dartx.codeUnitAt](i);
     }
@@ skipping 83 matching lines @@
   exports.JSString = JSString;
   exports.getInterceptor = getInterceptor;
   exports.JSBool = JSBool;
   exports.JSIndexable = JSIndexable;
   exports.JSMutableIndexable = JSMutableIndexable;
   exports.JSObject = JSObject;
   exports.JavaScriptObject = JavaScriptObject;
   exports.PlainJavaScriptObject = PlainJavaScriptObject;
   exports.UnknownJavaScriptObject = UnknownJavaScriptObject;
 });