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

lib/runtime/dart/typed_data.js

 dart_library.library('dart/typed_data', null, /* Imports */[
   "dart/_runtime",
   'dart/core'
 ], /* Lazy imports */[
   'dart/_native_typed_data'
 ], function(exports, dart, core, _native_typed_data) {
   'use strict';
   let dartx = dart.dartx;
   class ByteBuffer extends core.Object {}
   class TypedData extends core.Object {}
-  let _littleEndian = Symbol('_littleEndian');
+  const _littleEndian = Symbol('_littleEndian');
   class Endianness extends core.Object {
     _(littleEndian) {
       this[_littleEndian] = littleEndian;
     }
   }
   dart.defineNamedConstructor(Endianness, '_');
   dart.setSignature(Endianness, {
     constructors: () => ({_: [Endianness, [core.bool]]})
   });
   Endianness.BIG_ENDIAN = dart.const(new Endianness._(false));
@@ skipping 952 matching lines @@
   exports.Uint64List = Uint64List;
   exports.Float32List = Float32List;
   exports.Float64List = Float64List;
   exports.Float32x4List = Float32x4List;
   exports.Int32x4List = Int32x4List;
   exports.Float64x2List = Float64x2List;
   exports.Float32x4 = Float32x4;
   exports.Int32x4 = Int32x4;
   exports.Float64x2 = Float64x2;
 });