frog NPE when reporting missing arguments

[DartBot]

This issue was originally filed by [email protected]

---

Compiling this program will cause frog to NPE. (Problem seems to be trying to report span of arguments, but there are no arguments.)

void main() {
  foo();
}

void foo(int x) {
}
        ^
NullPointerException
    (TypeError: Cannot call method 'get$span' of undefined)
    at MethodMember.<anonymous> (/usr/local/google/home/mattsh/dart-code1/dart/frog/minfrog:5436:86)
    at MemberSet.<anonymous> (/usr/local/google/home/mattsh/dart-code1/dart/frog/minfrog:6083:35)
    at BareValue.<anonymous> (/usr/local/google/home/mattsh/dart-code1/dart/frog/minfrog:11878:19)
    at BareValue.invoke$4 (/usr/local/google/home/mattsh/dart-code1/dart/frog/minfrog:12138:15)
    at MethodGenerator.visitCallExpression (/usr/local/google/home/mattsh/dart-code1/dart/frog/minfrog:3435:19)
    at CallExpression.<anonymous> (/usr/local/google/home/mattsh/dart-code1/dart/frog/minfrog:10120:18)
    at MethodGenerator.visitValue (/usr/local/google/home/mattsh/dart-code1/dart/frog/minfrog:2963:20)
    at MethodGenerator.visitVoid (/usr/local/google/home/mattsh/dart-code1/dart/frog/minfrog:2981:15)
    at MethodGenerator.visitExpressionStatement (/usr/local/google/home/mattsh/dart-code1/dart/frog/minfrog:3353:20)
    at ExpressionStatement.<anonymous> (/usr/local/google/home/mattsh/dart-code1/dart/frog/minfrog:10071:18)

[dgrove]

Added Duplicate label.
Marked as being merged into #775.