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

pkg/dev_compiler/test/codegen_expected/closure.js

Index: pkg/dev_compiler/test/codegen_expected/closure.js
diff --git a/pkg/dev_compiler/test/codegen_expected/closure.js b/pkg/dev_compiler/test/codegen_expected/closure.js
index 67e09b961d0f069e410bd6c812a68a2cfc7a4846..7eabc56ffc8ba2d4c29fc99174f4f247d862d097 100644
--- a/pkg/dev_compiler/test/codegen_expected/closure.js
+++ b/pkg/dev_compiler/test/codegen_expected/closure.js
@@ -93,10 +93,10 @@ closure.Foo$ = dart.generic(T => {
   const v$ = Symbol("Foo.v");
   dart.setSignature(Foo, {
     fields: () => ({
-      i: core.int,
-      b: core.bool,
-      s: core.String,
-      v: T
+      i: dart.finalFieldType(core.int),
+      b: dart.fieldType(core.bool),
+      s: dart.fieldType(core.String),
+      v: dart.fieldType(T)
     }),
     getters: () => ({prop: dart.definiteFunctionType(core.String, [])}),
     setters: () => ({prop: dart.definiteFunctionType(dart.void, [core.String])}),