Closure Compiler issue with 3.0 preview

[RebeccaStevens]

When using the Closure Compiler with the 3.0 preview version of this element, Constant reassigned errors occur:

stdin:25174: ERROR - Constant reassigned: _lockedElementCache
  _lockedElementCache = [];
  ^^^^^^^^^^^^^^^^^^^

stdin:25175: ERROR - Constant reassigned: _unlockedElementCache
  _unlockedElementCache = [];
  ^^^^^^^^^^^^^^^^^^^^^

version: @polymer/iron-overlay-behavior#3.0.0-pre.10

Lines at fault

iron-overlay-behavior/iron-scroll-manager.js

Lines 102 to 103 in 1cbc71f

	export const _lockedElementCache = null;
	export const _unlockedElementCache = null;

If I change the lines above to use let instead of const, the compiler no longer complains.

[RebeccaStevens]

Related #267

[valdrinkoshi]

Looks like polymer-modulizer always converts object properties to const variables - @aomarks @FredKSchott could you take a look?