Fix null safety code generation and migrate to null safety

[GZGavinZhao]

This PR strives to achieve the following:

1. Make pageloader usable with Dart SDK 2.14+ and AngularDart 7+
2. To achieve 1, bump analyzer constraint (-> 2.8.0)
3. Fix the erroneous code generation when writing null safety code after 2 is done. The main problem is that the $WhateverPo class is not generated at all. To see this, rerun build_runner build and choose "Delete conflicting files".

A few notes:

1. I assume that before the null safety migration stopped for everything lib/src/generators was because the moment one changes the language version, the builder gets angry. The correct way to do this is to create a dummy project, copy the files under generators and api (the only thing generators depend on) over, bump the dependencies in the dummy project, and do the migration there.
2. Right now it's technically migrated, however code generation still doesn't work. PRs and all other suggestions are welcome.
3. There are a few places where I'm not sure what I'm supposed to do (e.g. DartType.displayName vs DartType.getDisplayString(withNullability: bool). All of these are marked with TODO(null-safety).

[GZGavinZhao]

Current failures:

[SEVERE] pageloader:pageloader on test/src/findchain.dart (cached):

Bad state: No element
[SEVERE] pageloader:pageloader on test/examples/correct_null_safety/generics.dart (cached):

type 'TypeParameterTypeImpl' is not a subtype of type 'ParameterizedType' in type cast
[SEVERE] pageloader:pageloader on test/examples/correct/list.dart (cached):

Bad state: No element
[SEVERE] pageloader:pageloader on test/examples/correct/generics.dart (cached):

type 'TypeParameterTypeImpl' is not a subtype of type 'ParameterizedType' in type cast
[SEVERE] pageloader:pageloader on test/examples/correct/iterables.dart (cached):

Bad state: No element
[SEVERE] Failed after 102ms

[GZGavinZhao]

The bad boy is analyzer 2.5.0, which is very strange because it has the most harmless CHANGELOG among all the versions!

[GZGavinZhao]

Closing in favor of #223.