[Cloud_firestore]add type params for invokeMethod calls.

[cyanglaz]

Description

Bump min flutter version to 1.2.0.
Add template type parameter for invokeMethod calls.
Updated invokeMethod and invokeMethod to using invokeListMethod and invokeMapMethod if any.
Anything that will change the type of a public API should not be included in this PR.

Related Issues

flutter/flutter#26431

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[collinjackson]

I wouldn't capitalize this

[collinjackson]

Capitalize Flutter

[collinjackson]

lgtm