This reorders some classes so that this file makes more sense, and adds
a bunch of docs. It also makes the following changes:

* Move allElements from Instrumentation to TestWidgets. (Instrumentation
  is going away.)

* Remove findElements.

* Rename byElement to byElementPredicate

* Rename byPredicate to byWidgetPredicate

* Implement _WidgetPredicateFinder so that byWidgetPredicate has good
  messages

* Fix one use of byElementPredicate to use byWidgetPredicate.

- Create a FlutterErrorDetails struct-like class that describes an

  exception along with more details that aren't in the exception, like

  where it was caught and what was going on when it was caught.



- Provide a FlutterError static API for handling these objects:



  - FlutterError.onError which is called whenever Flutter catches an

    error.



  - FlutterError.reportError() which handles an error.



  - FlutterError.dumpErrorToConsole() which is the default behavior

    for onError.



- Removes all the existing exception handler callbacks.



- Replaces all the existing places that described exceptions using

  debugPrint with calls to FlutterError.reportError().



- Extend lockState() to also catch exceptions, so that we catch

  exceptions that happen during finalizers.



- Make the test framework catch errors and treat them as failures.



- Provide a mechanism to override this behavior in the test framework.



- Make the tests that used to depend on the exception handler

  callbacks use this new mechanism.



- Make pump() also support the phase argument.



- Improve some tests using these new features.



Fixes #2356, #2988, #2985, #2220.

This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308

It has performance implications in release mode.

We'll enable the lint once https://github.com/dart-lang/linter/issues/195 is fixed.

Previously we used a positional argument for widgets that had multiple
children. Now we use a named argument that defaults to an empty list.

Fixes #241

This package contains WidgetTester, which is very useful when writing tests for
widgets.

- force the time dilation to 1.0 for the Widget tests, so that a local
  change doesn't break all the tests during development.
- add missing license block to all the files.
- set ui.window.onBeginFrame to null when you use WidgetTester, so that
  the engine doesn't trigger any confusing frames after our fake frames.

When we reactivate a subtree that had a global key, we weren't updating the
parent data because:

1) The child wasn't in the tree when we updated the parent data element.
2) The activated child didn't go through mount (just through update).

This patch moves the parent data update work to when we attach the render
object, which we do both during mount and when reactivating a child.

Fixes #345

Fixes #1084

Now that we've ported all the widget tests to fn3, we don't need a separate fn3
directory.