Merge pull request Tencent#1141 from Tencent/dev

update flutter sdk constraint

Merge pull request Tencent#1092 from Tencent/dev

for flutter v1.3.0

Merge pull request Tencent#1066 from Tencent/dev

for v1.2.16

Merge pull request Tencent#1009 from Tencent/dev

update flutter readme

Merge pull request Tencent#932 from Tencent/dev

update flutter example deps

Merge pull request Tencent#862 from Tencent/dev

for v1.2.13

Merge pull request Tencent#817 from Tencent/dev

upgrade Android & flutter deps

Merge pull request Tencent#779 from Tencent/dev

flutter unit test not ready

Merge pull request Tencent#710 from Tencent/dev

fix some javadoc warning

Update README.md