Component override is broken if the overridden component does not have the same theme states as the overriding one

[gbdrummer]

If we have this theme:

anchor {
  @state initial {
    color: green;
  }
}

anchor-button {
  @state initial {
    background: green;
    color: white;
  }

  @state focus {
    outline: 0;
  }
}

Because anchor-button overrides anchor, a build-blocking error will be thrown because anchor-button has a focus state in its theme while anchor does not. The diffing algo should allow for non-shared states.