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

test/codegen/expect/fieldtest.js

Index: test/codegen/expect/fieldtest.js
diff --git a/test/codegen/expect/fieldtest.js b/test/codegen/expect/fieldtest.js
index 49f33f6a72fe9a8e8dfd3097575dd8b78809c846..4d293909904520eca346e229ea04898ff01b1152 100644
--- a/test/codegen/expect/fieldtest.js
+++ b/test/codegen/expect/fieldtest.js
@@ -10,7 +10,7 @@ dart_library.library('fieldtest', null, /* Imports */[
       this.x = 42;
     }
   }
-  let B$ = dart.generic(function(T) {
+  const B$ = dart.generic(function(T) {
     class B extends core.Object {
       B() {
         this.x = null;
@@ -68,7 +68,7 @@ dart_library.library('fieldtest', null, /* Imports */[
     }
   }
   dart.virtualField(Derived, 'foo');
-  let Generic$ = dart.generic(function(T) {
+  const Generic$ = dart.generic(function(T) {
     class Generic extends core.Object {
       foo(t) {
         dart.as(t, T);