null type checking for numbers

[jmesserly]

the JS codegen would like to assume non-null for some types. This is especially true for numbers, but also may include other primitive types (bool, string). For arbitrary objects, it would also be nice to codegen obj.x ... otherwise we'd have to potentially check for null in some cases for that too. (we can likely avoid if the result has a further invocation)

[jmesserly]

I think @vsmenon implemented this now. so closing