VM doesn't always throw when expanding a list after adding an element

[leafpetersen]

Calling the List.length setter to expand the size of a list with non-nullable type should throw an error in null safe code. The VM sometimes fails to throw this exception if it follows a call to List.add. The code below shows this: the first call to the length setter throws, but after a call to List.add, subsequent calls do not throw, and null can be observed as the contents of a non-nullable list.

cc @nshahan @alexmarkov

void throws(void Function() f) {
  try {
    f();
  } catch (e, s) {
    return;
  }
  throw "Expected to throw";
}

void main() {
  List<int> x = [];
  throws(() => x.length = x.length + 1);
  print(x.length);
  x.add(222);
  print(x.length);
  x.length = 2;
  print(x.length);
  x.length = x.length + 1;
  print(x.length);
  for (int i = 0; i < x.length; i++) {
    print(x[i]);
  }
}

[leafpetersen]

Note that there are a some incorrectly migrated Dart 2 language tests which are passing only because of this failure, and will begin to fail once this issue is fixed. cc @munificent . Feel free to approve these failures and file an issue for @munificent and myself to triage and fix them (the fix depends somewhat on the resolution of this issue).