Bump play-service-maps dependency from version 18.1.0 to 18.2.0

[johnmccutchan]

Google Maps team has requested we update our client library version to the latest.

[flutter-dashboard]

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[gmackall]

nit: this should be "Updates...", per https://github.com/flutter/flutter/wiki/Contributing-to-Plugins-and-Packages#changelog-style

[johnmccutchan]

Done.

[gmackall]

Looks like there are some actual test failures here, the types used in the constructors of Polyline and Polygon seem to have changed. The tests seem to pass after just changing the respective types of the mocks used for these tests to fit

https://github.com/flutter/packages/blob/21a585f176a5e20028977ddfd49dafed0e87eecb/packages/google_maps_flutter/google_maps_flutter_android/android/src/test/java/io/flutter/plugins/googlemaps/PolygonControllerTest.java

https://github.com/flutter/packages/blob/21a585f176a5e20028977ddfd49dafed0e87eecb/packages/google_maps_flutter/google_maps_flutter_android/android/src/test/java/io/flutter/plugins/googlemaps/PolylineControllerTest.java

/b/s/w/ir/x/w/packages/packages/google_maps_flutter/google_maps_flutter_android/android/src/test/java/io/flutter/plugins/googlemaps/PolygonControllerTest.java:20: error: incompatible types: zzad cannot be converted to zzag
    final Polygon polygon = spy(new Polygon(z));
                                            ^
/b/s/w/ir/x/w/packages/packages/google_maps_flutter/google_maps_flutter_android/android/src/test/java/io/flutter/plugins/googlemaps/PolylineControllerTest.java:20: error: incompatible types: zzag cannot be converted to zzaj
    final Polyline polyline = spy(new Polyline(z));
                                               ^

, so zzad -> zzag and zzag -> zzaj