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

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

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

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

Don't give children negative sizes when overconstrained

One bug was masking the other, hence they both needed to be fixed
and tested separately.

@Hixie

Improve TextSpan

wait for text to change in the sample driver test

Now we just have one TextSpan class that handles both simple strings, trees of
children, and styling both. This approach simplifies the interface for most
clients.

This patch also removes StyledText, which was weakly typed and tricky to use
correctly. The replacement is RichText, which is strongly typed and uses
TextSpan.

Disable flaky retry_test

Keep AnimationController's _value in bounds

additional validation for device commands

Remove all inline templates and provide a way to specify templates as resources

Rather than clamping _value on read, we now clamp the value when writing it,
which simplifies reasoning about _value.

support waiting for things to happen in Flutter Driver