* Add structured errors in Animations, TabView, ChangeNotifier

* Add structured error on MaterialPageRoute, BoxBorder, DecorationImagePainter, TextSpan

* Add structured errors in Debug

* Fix test errors

* Add structured errors in Scaffold and Stepper

* Add structured errors in part of Rendering Layer

* Fix failing test due to FloatingPoint precision

* Fix failing tests due to precision error and not using final

* Fix failing test due to floating precision error with RegEx instead

* Add structured error in CustomLayout and increase test coverage

* Add structured error & its test in ListBody

* Add structured error in ProxyBox and increase test coverage

* Add structured error message in Viewport

* Fix styles and add more assertions on ErrorHint and DiagnosticProperty

* Add structured error in scheduler/binding and scheduler/ticker
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Add structured error in AssetBundle and TextInput
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Add structured errors in several widgets #1
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Remove unused import
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Add assertions on hint messages
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Fix catch spacing
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Add structured error in several widgets part 2 and increase code coverage
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Add structured error in flutter_test/widget_tester

* Fix floating precision accuracy by using RegExp
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Remove todo to add tests in Scaffold showBottomSheet
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Fix reviews by indenting lines and fixing the assertion orders
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Fix failing tests due to renaming class
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Try skipping the NetworkBundleTest
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Remove leading space in material/debug error hint
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Remove stray extra space

* Add ImageStreamCompleter.hasListeners (and cleanup)

This is mostly just some cleanup of stuff I ran into, but it makes
`hasListeners` protected on `ImageStreamCompleter`, because otherwise
there's no way to track if listeners are registered or not.

* Address review comments

Luckily this class didn't actually need to extend its superclass, it
only implements the interface. So we can change `extends` to
`implements` and that's close enough, while allowing the class to be
mixed in again.

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

I found that some ValueListeners want to know when they should start
doing work (e.g. if the value comes from polling a network resource).

It's common to have a ChangeNotifier that wraps a single value. This class
makes that easy by providing a generic implementation.

This patch aligns the iteration patterns used by animations and
ChangeNotifier. They now both respect re-entrant removal of listeners
and coalesce duplication registrations. (Also, ChangeNotifier
notification is no longer N^2).

Fixes #7533

It took me a while to figure out what was going on (I was removing a
listener after disposal). These asserts helped.

* MultiTapGestureRecognizer previously would assert if there was no
   competition.
 * GestureArenaTeam would always select the first recongizer as the
   winner even if a later recognizer actually accepted the pointer
   sequence.
 * debugPrintStack would fail a type check if maxFrames was non-null.
 * FractionalOffset.lerp would throw a null-pointer exception if its
   second argument was null.

Also, add a number of tests for previously untested lines of code.

This lets you use `Listenable.merge` without having to sanitize your
incoming list of change notifiers, in case your semantics are that
they are optional.

Sometimes you have several listenables, but you want to hand them to an
API (e.g. CustomPainter) that only expects one.

This patch improves some subtle behaviors about the change notifier.

This introduces the key parts of a paginated data table, not including
the built-in pagination features.

* Provide more data for the data table demo, so there's data to page.

* Introduce a ChangeNotifier class which abstracts out
  addListener/removeListener/notifyListeners. We might be able to use
  this to simplify existing classes as well, though this patch doesn't
  do that.

* Introduce DataTableSource, a delegate for getting data for data
  tables. This will also be used by ScrollingDataTable in due course.

* Introduce PaginatedDataTable, a widget that wraps DataTable and only
  shows N rows at a time, fed by a DataTableSource.