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

lib/runtime/dart/_js_mirrors.js

 dart_library.library('dart/_js_mirrors', null, /* Imports */[
   "dart/_runtime",
   'dart/_internal',
   'dart/core',
   'dart/mirrors'
 ], /* Lazy imports */[
 ], function(exports, dart, _internal, core, mirrors) {
   'use strict';
   let dartx = dart.dartx;
   function getName(symbol) {
@@ skipping 30 matching lines @@
   }
   dart.fn(_dload, dart.dynamic, [dart.dynamic, core.String]);
   function _dput(obj, name, val) {
     exports._dart.dput(obj, name, val);
   }
   dart.fn(_dput, dart.void, [dart.dynamic, core.String, dart.dynamic]);
   function _dsend(obj, name, args) {
     return exports._dart.dsend(obj, name, ...args);
   }
   dart.fn(_dsend, dart.dynamic, [dart.dynamic, core.String, core.List]);
-  let _toJsMap = Symbol('_toJsMap');
+  const _toJsMap = Symbol('_toJsMap');
   class JsInstanceMirror extends core.Object {
     _(reflectee) {
       this.reflectee = reflectee;
     }
     get type() {
       return dart.throw(new core.UnimplementedError("ClassMirror.type unimplemented"));
     }
     get hasReflectee() {
       return dart.throw(new core.UnimplementedError("ClassMirror.hasReflectee unimplemented"));
     }
@@ skipping 34 matching lines @@
   dart.setSignature(JsInstanceMirror, {
     constructors: () => ({_: [JsInstanceMirror, [core.Object]]}),
     methods: () => ({
       delegate: [dart.dynamic, [core.Invocation]],
       getField: [mirrors.InstanceMirror, [core.Symbol]],
       setField: [mirrors.InstanceMirror, [core.Symbol, core.Object]],
       invoke: [mirrors.InstanceMirror, [core.Symbol, core.List], [core.Map$(core.Symbol, dart.dynamic)]],
       [_toJsMap]: [dart.dynamic, [core.Map$(core.Symbol, dart.dynamic)]]
     })
   });
-  let _metadata = Symbol('_metadata');
-  let _declarations = Symbol('_declarations');
-  let _cls = Symbol('_cls');
+  const _metadata = Symbol('_metadata');
+  const _declarations = Symbol('_declarations');
+  const _cls = Symbol('_cls');
   class JsClassMirror extends core.Object {
     get metadata() {
       return this[_metadata];
     }
     get declarations() {
       return this[_declarations];
     }
     _(cls) {
       this[_cls] = cls;
       this.simpleName = core.Symbol.new(cls.name);
@@ skipping 152 matching lines @@
   }
   JsTypeMirror[dart.implements] = () => [mirrors.TypeMirror];
   dart.defineNamedConstructor(JsTypeMirror, '_');
   dart.setSignature(JsTypeMirror, {
     constructors: () => ({_: [JsTypeMirror, [core.Type]]}),
     methods: () => ({
       isAssignableTo: [core.bool, [mirrors.TypeMirror]],
       isSubtypeOf: [core.bool, [mirrors.TypeMirror]]
     })
   });
-  let _name = Symbol('_name');
+  const _name = Symbol('_name');
   class JsParameterMirror extends core.Object {
     _(name, t, annotations) {
       this[_name] = name;
       this.type = new JsTypeMirror._(t);
       this.metadata = core.List$(mirrors.InstanceMirror).from(annotations[dartx.map](dart.fn(a => new JsInstanceMirror._(a), JsInstanceMirror, [dart.dynamic])));
     }
     get defaultValue() {
       return dart.throw(new core.UnimplementedError("ParameterMirror.defaultValues unimplemented"));
     }
     get hasDefaultValue() {
@@ skipping 31 matching lines @@
     }
     get simpleName() {
       return dart.throw(new core.UnimplementedError("ParameterMirror.simpleName unimplemented"));
     }
   }
   JsParameterMirror[dart.implements] = () => [mirrors.ParameterMirror];
   dart.defineNamedConstructor(JsParameterMirror, '_');
   dart.setSignature(JsParameterMirror, {
     constructors: () => ({_: [JsParameterMirror, [core.String, core.Type, core.List]]})
   });
-  let _method = Symbol('_method');
-  let _params = Symbol('_params');
-  let _createParameterMirrorList = Symbol('_createParameterMirrorList');
+  const _method = Symbol('_method');
+  const _params = Symbol('_params');
+  const _createParameterMirrorList = Symbol('_createParameterMirrorList');
   class JsMethodMirror extends core.Object {
     _(cls, method) {
       this[_method] = method;
       this[_name] = getName(cls.simpleName);
       this[_params] = null;
       let ftype = exports._dart.classGetConstructorType(cls[_cls]);
       this[_params] = this[_createParameterMirrorList](ftype);
     }
     get constructorName() {
       return core.Symbol.new('');
@@ skipping 96 matching lines @@
   exports.getName = getName;
   exports.getSymbol = getSymbol;
   exports.reflect = reflect;
   exports.reflectType = reflectType;
   exports.JsInstanceMirror = JsInstanceMirror;
   exports.JsClassMirror = JsClassMirror;
   exports.JsTypeMirror = JsTypeMirror;
   exports.JsParameterMirror = JsParameterMirror;
   exports.JsMethodMirror = JsMethodMirror;
 });