SizeObserver crusade: Dismissable

Update engine revision

Remove examples/fitness

We originally wrote examples/fitness to be an app which all
members of the Flutter team could carry on their phones and
use every day.  It served us well for testing of Text input,
Keyboards and writing/reading JSON.  It's never actually become
a real carry app, so lets just let it go for now.

@abarth @collinjackson

Ignore `flutter analyze` errors via analysis options.

Refactor Dismissable to not need a SizeObserver and to generally tighten
up the code a bit.

# Conflicts:
#	packages/flutter_tools/lib/src/commands/analyze.dart

Migrates error suppression from regexps to analysis options.
* Ignore TODOs.
* Ignore strong mode field and method overrides (see `InvalidFieldOverride` and `InvalidMethodOverride` in `strong/info.dart`.
* Remove stale regexp to capture refs to dead `dart:ui_internals` URI.
* Remove stale regexp to capture dead strong mode "will need runtime check to cast to type" error (https://github.com/dart-lang/sdk/issues/24542).

5 regexp downs, another small handful (~4) to go! :)

Do not create a font manifest if the app has no flutter.yaml

Migrates error suppression from regexps to analysis options.
* Ignore TODOs.
* Ignore strong mode field and method overrides (see `InvalidFieldOverride` and `InvalidMethodOverride` in `strong/info.dart`.
* Remove stale regexp to capture refs to dead `dart:ui_internals` URI.
* Remove stale regexp to capture dead strong mode "will need runtime check to cast to type" error (https://github.com/dart-lang/sdk/issues/24542).

5 regexp downs, another small handful (~4) to go! :)

Pulls in the following changed required for iOS:
* Application identifiers are legal UTIs.
* Dart precompiled dylib is not a candidate for dead code stripping.
* Account for relative paths on readlink in SnapshotterInvoke

Re-run `flutter create` on examples to fill in icons and other assets

Use pointycastle as crypto library. cipher is deprecated.

Update android_workflow.dart

SizeObserver crusade: ScrollableViewport and tabs

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

Add flipped curved

fix https://github.com/flutter/flutter/issues/2157

add an android icon to the starter app

fix a regression in the android manifest

Remove PointerRouter and GestureArena arguments

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.

run flutter doctor as part of create

Add the ability to recognize gestures on text spans

Fix move() to call markNeedsLayout().

Otherwise, changing the order of your children (e.g. shuffling a list of
children) will have no visible effect.

Currently the interface for recognizing gestures on text spans is pretty ugly,
but hopefully we can improve it with time.

Fixes #156

Fix TextSpan's operator==

We forgot to compare the lengths of the lists.

Just-in-time mutations of GestureDetector