See https://github.com/flutter/flutter/pull/6861

This silences all but two of the warnings from https://travis-ci.org/flutter/flutter/builds/176055550

One of the silenced warnings was a genuine code error.

Some of the others were correct but there was no way for the analyzer to know, and I worked around them.

The remainder are problems with the analyzer, specifically https://github.com/dart-lang/sdk/issues/27827 and https://github.com/dart-lang/sdk/issues/27504.

* Udpate to Dart `1.20.0-dev.1.0`.

Udpate to Dart `1.20.0-dev.1.0` and corresponding `analyzer` package.

* Added `--no-packages-dir`.

* Revert extension to master.

* Added ignores.

* Review nits.

* More nits.

Fixes https://github.com/flutter/flutter/issues/5380

Now that protected can be accessed from inside the same library, we can
use protected in a number of new places and we can remove some
trampolines we were using to work around its previous semantics.

We have these details objects for the same reason we now have drag details
objects: future extensibility.

Also, add more dartdoc to widgets.dart.

Previously we supplied individual parameters to the various drag and pan
callbacks. However, that approach isn't extensible because each new
parameter is a breaking change to the API.

This patch makes a one-time breaking change to the API to provide a
"details" object that we can extend over time as we need to expose more
information. The first planned extension is adding enough information to
accurately produce an overscroll glow on Android.

Also, make some previously private classes public for better
documentation.

Also, add some missing docs to http.dart and widgets.dart.

The old names were getting silly and started stepping on valuable namespace.

The new names are consistent and clear.

Fixes #2723

This patch adds a consistent, simple description for the common pattern of
having a single child widget.

The problem was we were using a tap gesture to stop the motion of the
drawer and a drag gesture to settle it. That can cause a broken
lifecycle. Now we use a single drag recognizer to drive the whole
lifecycle.

Fixes #775
Fixes #1276

This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308

Fixes #1482

We'll need this for RTL support because the RTL state will live in the widget
tree. Also, remove the `oldWidget` argument to updateRenderObject because there
aren't any clients for it.

This makes WidgetError more like RenderingError, which will aid with https://github.com/flutter/flutter/issues/2356.

Fixes https://github.com/flutter/flutter/issues/2443

Fixes #1381

This assert was overzealous. You can update a RawGestureDetector any
time before the semantics phase, it doesn't have to only be during your
child's layout.

This is good because Block has additional PointerListeners between its
GestureDetector and its Viewport.

Also:
 - add operator==/hashCode/toString to ViewportDimensions
 - add toString to BindingBase
 - add toString and debugFillDescription to ScrollBehavior
 - fix a bug in the RawGestureDetectorState's replaceGestureRecognizers
 - rename MixedViewport's onExtentsUpdate to onExtentChanged
 - replace ExtentsUpdateCallback with ValueChanged<double>
 - remove a microtask for dispatching scroll start, since it
   did not appear to have any purpose
 - added dartdocs to Instrumentation until I understood it
 - made all event dispatch in Instrumentation drain microtasks

There's no reason to make clients supply a PointerRounter and a
GestureArena when constructing gesture recognizers. These objects are
statics and the gesture recognizers can just grab them directly.

Also, remove the callback constructor arguments. Almost no code used
them. Instead, people seem to prefer using the `..` operator to set
callbacks on the recognizers. Removing the arguments removes a bunch of
boilerplate.

This allows us to adjust exactly which gestures we're listening for
during layout, which I'll use to kill a SizeObserver.

We were using an Offset, which represented pixels/second, but it wasn't
clear to clients whether that was pixels/ms. Now we use a Velocity class
that is explict about the units.

Fixes #1510
Fixes #785

... just a start.

Instead of PointerInputEvent having a "type" field, we now have a
different class for each pointer type.

This has ripple effects throughout the system.

I also did code cleanup in affected files while I was there.

Elaborate on the debugging information exposed by
RenderPointerListener, Listener, and GestureDetector.

Draggable is now itself a gesture arena member. This means it won't
conflict with other gesture recognisers in the same path.

This also allows variants of Draggable that are triggered by other
gestures.

Also, some cleanup of DoubleTapGestureRecognizer, GestureDetector, and
PrimaryPointerGestureRecognizer.

Also, make MultiTapGestureRecognizer support a timeout for longpress.

Also, make Draggable data be typed.

Also, hide warnings about constructor warnings for now. Analyzer doesn't
support them yet. (Have to do this on a per-line basis)

Directions for future research:
 - animating the avatar (enter/exit transitions)
 - interaction with the navigator (canceling a drag on page navigation, etc)
 - double-tap draggable

A translucent gesture detector still listens for gestures but also lets the
content visually behind the detector receive events.