Index flow of nullable back to upstream through a field write

[nimakarimipour]

This PR is built on (#210 landed)

Resolves #209 by indexing flow of nullable back to upstream through field writes. Previously we only indexed flow of nullable back to upstream via parameter passing, e.g. code below:

class Target{
    Object getFoo()
    void bar(Object param);
}
class Dependency {
    Target t;
    void baz(){
       t.bar(t.getFoo());
    }
}

We know that making getFoo() will trigger a fix on bar() parameter. This PR updates Annotator to index all flows back to upstream including field writes. Hence, in the code below:

class Target{
    Object bar;
    Object getFoo();
}
class Dependency {
    Target t;
    void baz(){
       t.bar = t.getFoo();
    }
}

Annotator now indexes the flow of nullable back to target through write to a field and includes the corresponding fix in the processing fix tree.

Please note this PR only updates indexing flow of nullable back to upstream. Annotator still does not index the impact of making public fields @Nullable on downstream dependencies.

[msridhar]

Please note this PR only updates indexing flow of nullable back to upstream. Annotator still does not index the impact of making public fields @Nullable on downstream dependencies.

I don't understand the importance of this. Can not indexing the impact of making public fields @Nullable lead to similar bugs?

[msridhar]

I did not deeply understand the changes, but from what I can tell, this bug was fixed mostly by deleting code and a bit of generalization, which is great!

[nimakarimipour]

Please note this PR only updates indexing flow of nullable back to upstream. Annotator still does not index the impact of making public fields @Nullable on downstream dependencies.

I don't understand the importance of this. Can not indexing the impact of making public fields @Nullable lead to similar bugs?

@msridhar The bug here was that we were flowing back to upstream through field writes and Annotator was not extending the tree accordingly. Making fields @Nullable on upstream can potentially cause problems in downstream dependencies but Annotator is not aware of that and will not enter an unexpected state. Please note in strict mode, we do not re-build all downstream dependencies to ensure no new bug has been introduced. We on rely on the errors indexed in the downstream scan phase which happens prior to annotating process.