Revert "Started looking for the dart plugin registrant at runtime (#31418)

[gaaclarke]

This reverts commit e97c801.

DO NOT LAND UNTIL THE REVERT OF flutter/flutter#98046

reason: Performance regression of ~275ms startup time in google app. b/224294947

workaround: specify the library uri explicitly of you custom entrypoint.

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[flutter-dashboard]

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