Fix: Combine initialization

[Maksimka101]

Did the following fixes:

• the BackgroundIsolateBinaryMessenger initialization

PR Checklist

• Code is 100% covered.
• I updated/added relevant documentation.
• All existing and new tests are passing.
• Changelog is updated.
• Version is pumped

[codecov-commenter]

Codecov Report

Merging #18 (c383946) into master (24188b0) will not change coverage.
Report is 1 commits behind head on master.
The diff coverage is 100.00%.

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@@            Coverage Diff            @@
##            master       #18   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           30        28    -2     
  Lines          383       381    -2     
=========================================
- Hits           383       381    -2     

Files Changed	Coverage Δ
...indings/mock_binary_messenger_service_binding.dart	100.00% <100.00%> (ø)
...ib/src/isolate_factory/native_isolate_factory.dart	100.00% <100.00%> (ø)