BaseNumberConverter should be effectively sealed

[ericstj]

See dotnet/corefx#37278

BaseNumberConverter shouldn't be derivable. On desktop it's a compile error. Since netstandard.dll doesn't include internal members it permits compile, but fails at runtime for most frameworks since internal abstract members are not overridden.

/cc @Anipik

[ericstj]

As mentioned here there are more types that fell under this issue: #1176 (comment)