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

pkg/dev_compiler/test/codegen_expected/sunflower/sunflower.js

 define(['dart_sdk'], function(dart_sdk) {
   'use strict';
   const core = dart_sdk.core;
   const html = dart_sdk.html;
   const math = dart_sdk.math;
   const dart = dart_sdk.dart;
   const dartx = dart_sdk.dartx;
   const sunflower = Object.create(null);
   const circle = Object.create(null);
   const painter = Object.create(null);
@@ skipping 83 matching lines @@
       this[x$] = x;
       this[y$] = y;
       this[radius$] = radius;
     }
   };
   const x$ = Symbol("Circle.x");
   const y$ = Symbol("Circle.y");
   const radius$ = Symbol("Circle.radius");
   dart.setSignature(circle.Circle, {
     fields: () => ({
-      x: core.num,
-      y: core.num,
-      radius: core.num
+      x: dart.finalFieldType(core.num),
+      y: dart.finalFieldType(core.num),
+      radius: dart.finalFieldType(core.num)
     })
   });
   painter.CirclePainter = class CirclePainter extends core.Object {
     new() {
       this[color] = painter.ORANGE;
     }
     get color() {
       return this[color];
     }
     set color(value) {
       this[color] = value;
     }
     draw(context) {
       context.beginPath();
       context.lineWidth = 2;
       context.fillStyle = this.color;
       context.strokeStyle = this.color;
       context[dartx.arc](this.x, this.y, this.radius, 0, painter.TAU, false);
       context[dartx.fill]();
       context.closePath();
       context.stroke();
     }
   };
   const color = Symbol("CirclePainter.color");
   painter.CirclePainter[dart.implements] = () => [circle.Circle];
   dart.setSignature(painter.CirclePainter, {
-    fields: () => ({color: core.String}),
+    fields: () => ({color: dart.fieldType(core.String)}),
     methods: () => ({draw: dart.definiteFunctionType(dart.void, [html.CanvasRenderingContext2D])})
   });
   sunflower.SunflowerSeed = class SunflowerSeed extends dart.mixin(circle.Circle, painter.CirclePainter) {
     new(x, y, radius, color) {
       if (color === void 0) color = null;
       super.new(x, y, radius);
       if (color != null) this.color = color;
     }
   };
   dart.defineLazy(painter, {
@@ skipping 29 matching lines @@
   }, null);
   // Exports:
   return {
     sunflower: sunflower,
     circle: circle,
     painter: painter
   };
 });
 
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