fix(toolkit): doesn't load on platforms without the Disposable API

[rix0rrr]

Toolkit classes reference the public static Symbol.asyncDispose symbol, but that is only defined on runtimes that are implementing the new Disposable API yet.

On platforms that don't know that API, that constant doesn't exist and the class definition fails.

Polyfill it.

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.47%. Comparing base (445fd71) to head (f46da2f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #309      +/-   ##
==========================================
- Coverage   85.54%   85.47%   -0.07%     
==========================================
  Files         222      222              
  Lines       36926    36926              
  Branches     4458     4452       -6     
==========================================
- Hits        31588    31563      -25     
- Misses       5248     5267      +19     
- Partials       90       96       +6     

Flag	Coverage Δ
suite.unit	85.47% <ø> (-0.07%)	⬇️

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