In Enzyme v3, I saw many issues about changes that we have to call update() after instance.setState().

Actually, wrapper.update() doesn't rerender even though the name implies to update something.
wrapper.update() only does to get a latest ReactElementTree from ReactShallowRenderer.
I think it would be nice if developers don't care about calling wrapper.update().

• https://github.com/airbnb/enzyme/blob/master/docs/guides/migration-from-2-to-3.md#for-mount-updates-are-sometimes-required-when-they-werent-before

In this case, enzyme can't handle all cases that need to call wrapper.update().

expect(wrapper.find('p').text()).to.equal('0');
wrapper.find('.button').prop('onClick'); // Update the value to '1'
// enzyme should call wrapper.update() but enzyme can't handle that
// because the above calls ReactComponent instance's setState directly 
expect(wrapper.find('p').text()).to.equal('0');
wrapper.update(); // Get the latest nodes from ShallowRenderer
expect(wrapper.find('p').text()).to.equal('1');

In order to resolve this, this PR adopts a pull-based approach, which calls wrapper.update() in getter for internal nodes.
So we can get the latest node through the enzyme APIs (find, text, ...).

expect(wrapper.find('p').text()).to.equal('0');
wrapper.find('.button').prop('onClick'); // Update the value to '1'
// wrapper has nodes that are before updating.
expect(wrapper.find('p')/* Get the latest nodes from ShallowRenderer */.text()).to.equal('1');

This PR is for ShallowWrapper, but I could apply this to ReactWrapper as well.

But there is a limitation.
This PR doesn't cover that a wrapper isn't RootShallowWrapper.

let child = wrapper.find('p');
expect(child.text()).to.equal('0');
child.prop('onClick'); // Update the value to '1'
expect(child).to.equal('0');
// we have to get the child from the wrapper again
let child = wrapper.find('p');
expect(child).to.equal('1');

Covering the case might be difficult because enzyme has to get child from the updated RootShallowWrapper with the same condition.
But enzyme has to care about a case that can't get child due to the update.
I think it seems to be good that wrappers that are not RootShallowWrapper treat as immutable.

BTW. ShallowWrapper has the condition(this[ROOT] === this) in many methods, I think it becomes clearer if ShallowWrapper can be separated RootShallowWrapper and ChildShallowWrapper.