Add support for indexing of impacts of changes on fields on downstream dependencies

[nimakarimipour]

This PR adds support for indexing impact of changes on fields on downstream dependencies. Prior to this PR, Annotator might make a field @Nullable that can trigger errors on downstream dependencies, with this PR, Annotator is aware of these impacts and if changes on fields, create an unresolvable error on downstream dependencies, the containing fix tree is rejected.

See example below:

On Target

public class Foo {

  public Object f0;

  public Object f1;

  public Object f2 = new Object();
}

On Dependency (Dep)

package test.depa;

import test.target.Foo;

public class Dep {

    public void directAccess(Foo foo){
         // making f0 will create an unresolvable error here.
         foo.f0.toString();
    }

    public void flowF1BackToF2(Foo foo) {
      // If f1 is annotated as @Nullable, the corresponding error on f2 can be resolved on target.
      foo.f2 = foo.f1;
    }
}

Prior to this PR, Annotator would have made f0 @Nullable even thought it causes an unresolvable error in Dep module. With this PR,

• Annotator is aware of this impact and will avoid making f0 @Nullable
• Annotator is aware that the triggered error by making f1 @Nullable is resolvable by making f2 @Nullable, therefore it includes the corresponding fix (f2) in any tree containing f1.

[msridhar]

Overall looks really good! Just a couple of comment

[msridhar]

Should this comment be before line 90?

[nimakarimipour]

Thanks for catching this. Fixed it in 15c8e1e

[msridhar]

This method is doing something different than the docs. It seems to look for a single field declaration that declares all the names in the Set given as a parameter. We should update the docs and consider renaming the method.

[nimakarimipour]

@msridhar Yes, I agree. Regarding field declarations (or OnFIeld instances) we have to document somewhere how they are created. Throughout the entire core module, every OnField instance contains all inline field names for the declaration, for instance for a declaration Foo a, b, c, all OnField instances targeting this declaration, contain all a, b and c on their variables property. This has been done at deserializing fixes here. That is the reason why I used equals instead of checking if it contains it. Will change the name and the documentation now.

[nimakarimipour]

Updated javadoc for this method 71b901d.
Will work on a more detailed documentation with revisiting all methods working on fields in a followup PR.

[msridhar]

I think this is modifying logic introduced in #220. Still not sure why we are creating an empty file here.

[nimakarimipour]

Yes, this is fixed in #220 now.

[msridhar]

LGTM! Just one very minor comment, can land after that