a few small refactorings to closure workarounds

lib/runtime/dart/collection.js

 dart_library.library('dart/collection', null, /* Imports */[
   'dart/_runtime',
   'dart/core'
 ], /* Lazy imports */[
   'dart/_internal',
   'dart/_js_helper',
   'dart/math'
 ], function(exports, dart, core, _internal, _js_helper, math) {
   'use strict';
   let dartx = dart.dartx;
@@ skipping 95 matching lines @@
         new: [HashMap$(K, V), [], {equals: dart.functionType(core.bool, [K, K]), hashCode: dart.functionType(core.int, [K]), isValidKey: dart.functionType(core.bool, [core.Object])}],
         identity: [HashMap$(K, V), []],
         from: [HashMap$(K, V), [core.Map]],
         fromIterable: [HashMap$(K, V), [core.Iterable], {key: dart.functionType(K, [dart.dynamic]), value: dart.functionType(V, [dart.dynamic])}],
         fromIterables: [HashMap$(K, V), [core.Iterable$(K), core.Iterable$(V)]]
       })
     });
     return HashMap;
   });
   let HashMap = HashMap$();
+  const _newSet = Symbol('_newSet');
   const SetMixin$ = dart.generic(function(E) {
     class SetMixin extends core.Object {
       [Symbol.iterator]() {
         return new dart.JsIterator(this.iterator);
       }
       get isEmpty() {
         return this.length == 0;
       }
       get isNotEmpty() {
         return this.length != 0;
@@ skipping 301 matching lines @@
         return IterableBase.iterableToFullString(set, '{', '}');
       }
     }
     dart.setSignature(SetBase, {
       statics: () => ({setToString: [core.String, [core.Set]]}),
       names: ['setToString']
     });
     return SetBase;
   });
   let SetBase = SetBase$();
-  const _newSet = Symbol('_newSet');
   const _HashSetBase$ = dart.generic(function(E) {
     class _HashSetBase extends SetBase$(E) {
       difference(other) {
         let result = this[_newSet]();
         for (let element of this) {
           if (!dart.notNull(other.contains(element))) result.add(element);
         }
         return result;
       }
       intersection(other) {
@@ skipping 5427 matching lines @@
   exports.LinkedHashMapKeyIterable$ = LinkedHashMapKeyIterable$;
   exports.LinkedHashMapKeyIterable = LinkedHashMapKeyIterable;
   exports.LinkedHashMapKeyIterator$ = LinkedHashMapKeyIterator$;
   exports.LinkedHashMapKeyIterator = LinkedHashMapKeyIterator;
   exports.HashSetIterator$ = HashSetIterator$;
   exports.HashSetIterator = HashSetIterator;
   exports.LinkedHashSetCell = LinkedHashSetCell;
   exports.LinkedHashSetIterator$ = LinkedHashSetIterator$;
   exports.LinkedHashSetIterator = LinkedHashSetIterator;
 });