[Flutter Driver] Simplified the serialization/deserialization logic of the Descendant/…

[adazh]

…Ancestor matchers.

Instead of flattening the "of" and "matching" matchers to the JSON string, using jsonEncode/jsonDecode to convert the matchers to String/from String.

This will make the serialization in Espresso/Java much simpler.

Description

Updated the serialization/deserialization logic for the Descendant/Ancestor matchers. Take the "get_diagnostics_tree" for example.
Before:
{"id":"message-7","method":"ext.flutter.driver","params":{"isolateId":"isolates/4120307314110843","of_keyValueType":"String","of_keyValueString":"TwoTimesCounterContainer","of_finderType":"ByValueKey","matching_type":"Text", "matcher_finderType":
"ByType","finderType":"Descendant","diagnosticsType":"widget","includeProperties":true,"subtreeDepth":20,"command":"get_diagnostics_tree","timeout":10000},"jsonrpc":"2.0"}

After:
{"id":"message-7","method":"ext.flutter.driver","params":{"isolateId":"isolates/4120307314110843","of":"{\"keyValueType\":\"String\",\"keyValueString\":\"TwoTimesCounterContainer\",\"finderType\":\"ByValueKey\"}","matching":"{
\"type\":\"Text\",\"finderType
":\"ByType\"}","finderType":"Descendant","diagnosticsType":"widget","includeProperties":true,"subtreeDepth":20,"command":"get_diagnostics_tree","timeout":10000},"jsonrpc":"2.0"}

Tests

flutter_test test/src/extension_test.dart still passes.

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Breaking Change

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[fluttergithubbot]

It looks like this pull request may not have tests. Please make sure to add tests before merging. While there are exceptions to this rule, if this patch modifies code it is probably not an exception.

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[goderbauer]

This didn't require any changes to tests?

[adazh]

Ah, forgot about the find_test.dart. On it.

[adazh]

This didn't require any changes to tests?

Tests are updated.

[goderbauer]

LGTM

Technically, this is a breaking change for anybody using the API, but that's probably just flutter driver and you?

[goderbauer]

nit: indentation.

[adazh]

LGTM

Technically, this is a breaking change for anybody using the API, but that's probably just flutter driver and you?

Yep, I think so.