fix #29182, generate top level const fields lazily

pkg/dev_compiler/test/codegen_expected/map_keys.js

 define(['dart_sdk'], function(dart_sdk) {
   'use strict';
   const core = dart_sdk.core;
   const math = dart_sdk.math;
   const dart = dart_sdk.dart;
   const dartx = dart_sdk.dartx;
   const map_keys = Object.create(null);
   let VoidTodynamic = () => (VoidTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [])))();
   map_keys.main = function() {
-    core.print(dart.map({'1': 2, '3': 4, '5': 6}));
-    core.print(dart.map([1, 2, 3, 4, 5, 6]));
-    core.print(dart.map({'1': 2, [dart.str`${dart.notNull(math.Random.new().nextInt(2)) + 2}`]: 4, '5': 6}));
+    core.print(dart.map({'1': 2, '3': 4, '5': 6}, core.String, core.int));
+    core.print(dart.map([1, 2, 3, 4, 5, 6], core.int, core.int));
+    core.print(dart.map({'1': 2, [dart.str`${dart.notNull(math.Random.new().nextInt(2)) + 2}`]: 4, '5': 6}, core.String, core.int));
     let x = '3';
-    core.print(dart.map(['1', 2, x, 4, '5', 6]));
-    core.print(dart.map(['1', 2, null, 4, '5', 6]));
+    core.print(dart.map(['1', 2, x, 4, '5', 6], core.String, core.int));
+    core.print(dart.map(['1', 2, null, 4, '5', 6], core.String, core.int));
   };
   dart.fn(map_keys.main, VoidTodynamic());
   dart.trackLibraries("map_keys", {"map_keys.dart": map_keys}, null);
   // Exports:
   return {
     map_keys: map_keys
   };
 });
 
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