Relax type checks based on strong mode guarantees

From 1.1.0, please make sure code using built_collection passes strong mode checks:

https://github.com/dart-lang/dev_compiler/blob/master/STRONG_MODE.md

For example, this used to need a runtime check:

new BuiltList((b) => b..add('not an int'))

Now it can be caught via static analysis, so the runtime check has been removed to improve performance.