fix #27258, don't allow dynamic set of a final field

pkg/dev_compiler/test/codegen_expected/BenchmarkBase.js

 define(['dart_sdk'], function(dart_sdk) {
   'use strict';
   const core = dart_sdk.core;
   const dart = dart_sdk.dart;
   const dartx = dart_sdk.dartx;
   const BenchmarkBase$ = Object.create(null);
   let VoidToNull = () => (VoidToNull = dart.constFn(dart.definiteFunctionType(core.Null, [])))();
   BenchmarkBase$.Expect = class Expect extends core.Object {
     static equals(expected, actual) {
       if (!dart.equals(expected, actual)) {
@@ skipping 65 matching lines @@
       this.teardown();
       return result;
     }
     report() {
       let score = this.measure();
       core.print(dart.str`${this.name}(RunTime): ${score} us.`);
     }
   };
   const name$ = Symbol("BenchmarkBase.name");
   dart.setSignature(BenchmarkBase$.BenchmarkBase, {
-    fields: () => ({name: core.String}),
+    fields: () => ({name: dart.finalFieldType(core.String)}),
     methods: () => ({
       run: dart.definiteFunctionType(dart.void, []),
       warmup: dart.definiteFunctionType(dart.void, []),
       exercise: dart.definiteFunctionType(dart.void, []),
       setup: dart.definiteFunctionType(dart.void, []),
       teardown: dart.definiteFunctionType(dart.void, []),
       measure: dart.definiteFunctionType(core.double, []),
       report: dart.definiteFunctionType(dart.void, [])
     }),
     statics: () => ({measureFor: dart.definiteFunctionType(core.double, [core.Function, core.int])}),
     names: ['measureFor']
   });
   dart.trackLibraries("BenchmarkBase", {
     "BenchmarkBase.dart": BenchmarkBase$
   }, null);
   // Exports:
   return {
     BenchmarkBase: BenchmarkBase$
   };
 });