Uncomment Marker icons now that ImageListener API change has landed in stable

[timsneath]

Description

This change resolves issue flutter/flutter#33438.

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[cyanglaz]

LGTM! Thanks. Probably need rebase tho :)

[stuartmorgan]

@timsneath Are you planning on updating this? I just checked and it still applies, so we should get it rebased and landed.

[timsneath]

Oh wow -- yeah, I can reland this.

[stuartmorgan]

Merged in the latest master, updated for NNBD, and removed the version bump since it's example-only (so doesn't really affect the published version).

[fluttergithubbot]

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