fix #569, cache constants defined in methods

test/codegen/expect/DeltaBlue.js

 dart_library.library('DeltaBlue', null, /* Imports */[
   'dart_sdk'
 ], function(exports, dart_sdk) {
   'use strict';
   const core = dart_sdk.core;
   const dart = dart_sdk.dart;
   const dartx = dart_sdk.dartx;
   const DeltaBlue$ = Object.create(null);
   const BenchmarkBase$ = Object.create(null);
   DeltaBlue$.main = function() {
@@ skipping 63 matching lines @@
       super.BenchmarkBase("DeltaBlue");
     }
     run() {
       DeltaBlue$.chainTest(100);
       DeltaBlue$.projectionTest(100);
     }
   };
   dart.setSignature(DeltaBlue$.DeltaBlue, {
     constructors: () => ({DeltaBlue: [DeltaBlue$.DeltaBlue, []]})
   });
+  let const$;
   DeltaBlue$.Strength = class Strength extends core.Object {
     Strength(value, name) {
       this.value = value;
       this.name = name;
     }
     nextWeaker() {
-      return dart.const(dart.list([DeltaBlue$.STRONG_PREFERRED, DeltaBlue$.PREFERRED, DeltaBlue$.STRONG_DEFAULT, DeltaBlue$.NORMAL, DeltaBlue$.WEAK_DEFAULT, DeltaBlue$.WEAKEST], DeltaBlue$.Strength))[dartx.get](this.value);
+      return (const$ || (const$ = dart.const(dart.list([DeltaBlue$.STRONG_PREFERRED, DeltaBlue$.PREFERRED, DeltaBlue$.STRONG_DEFAULT, DeltaBlue$.NORMAL, DeltaBlue$.WEAK_DEFAULT, DeltaBlue$.WEAKEST], DeltaBlue$.Strength))))[dartx.get](this.value);
     }
     static stronger(s1, s2) {
       return dart.notNull(s1.value) < dart.notNull(s2.value);
     }
     static weaker(s1, s2) {
       return dart.notNull(s1.value) > dart.notNull(s2.value);
     }
     static weakest(s1, s2) {
       return dart.notNull(DeltaBlue$.Strength.weaker(s1, s2)) ? s1 : s2;
     }
@@ skipping 496 matching lines @@
     statics: () => ({
       equals: [dart.void, [dart.dynamic, dart.dynamic]],
       listEquals: [dart.void, [core.List, core.List]]
     }),
     names: ['equals', 'listEquals']
   });
   // Exports:
   exports.DeltaBlue = DeltaBlue$;
   exports.BenchmarkBase = BenchmarkBase$;
 });