[web] Fix hot restart issues

[MatthaiosSait]

@JulianBissekkou
The problem:

When doing a hot restart, there comes an error in the console,
WARNING: Too many active WebGL contexts. Oldest context will be lost., and
WebGL: INVALID_OPERATION: delete: object does not belong to this context.

Also, this happens when you open multiple Map instances. You can test it by navigating back and forth to a new map instance. After about 8 times the same error will occur and the map will crash.

The second use case is in my opinion more important than the hot restart use case.

[MatthaiosSait]

With flutter/flutter#124334, they will be fixed once and for all. Until, then, an intermediate fix will be provided.