native impl parameter names not validated against header

[jvasileff]

For example, this typechecks:

native class NC3(String s) {
    native shared void foo() => print(s);
}

native("js") class NC3(String t) {}
native("jvm") class NC3(String t) {}

shared void run() {
    NC3("hello").foo();

    NC3 {
        s = "hello";
    }.foo();
}

but prints <null> <null> on JS and results in a backend error on the JVM.