Fix #28120, strong mode allows field overrides

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 47 matching lines @@
       let theta = i * painter.TAU / dart.notNull(sunflower.PHI);
       let r = dart.notNull(math.sqrt(i)) * sunflower.SCALE_FACTOR;
       let x = sunflower.centerX + r * dart.notNull(math.cos(theta));
       let y = sunflower.centerY - r * dart.notNull(math.sin(theta));
       new sunflower.SunflowerSeed(x, y, sunflower.SEED_RADIUS).draw(sunflower.context);
     }
     sunflower.notes[dartx.text] = dart.str`${sunflower.seeds} seeds`;
   };
   dart.fn(sunflower.draw, VoidTovoid());
   circle.Circle = class Circle extends core.Object {
+    get x() {
+      return this[x$];
+    }
+    set x(value) {
+      super.x = value;
+    }
+    get y() {
+      return this[y$];
+    }
+    set y(value) {
+      super.y = value;
+    }
+    get radius() {
+      return this[radius$];
+    }
+    set radius(value) {
+      super.radius = value;
+    }
     new(x, y, radius) {
-      this.x = x;
-      this.y = y;
-      this.radius = radius;
+      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
     })
   });
   painter.CirclePainter = class CirclePainter extends core.Object {
     new() {
-      this.color = painter.ORANGE;
+      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}),
     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;
@@ skipping 20 matching lines @@
   dart.trackLibraries("sunflower", {"sunflower.dart": sunflower, "circle.dart": circle, "painter.dart": painter}, null);
   // Exports:
   return {
     sunflower: sunflower,
     circle: circle,
     painter: painter
   };
 });
 
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