More BoxConstraints asserts

The asserting will continue until morale improves!

* Convert all assert(*.isNormalized) checks to use the new
  debugAssertIsNormalized technology.
* Convert CustomMultiChildLayout to use the new RenderingError
  technology to greatly improve the detail in errors you get when
  writing CustomMultiChildLayout delegates.
* Add BoxConstraints.copyWith().
* Indent the descendants in the rendering exception data dump so that
  when you have multiple children it's clearer what's going on.

ScrollBehavior should support negative min scroll offset

Fixes #1730

Refactor service-related code to be shared between android and IOS.

Improve asserts around BoxConstraints

The IOS code is totally untested. Need a Mac to test it.

Negative constraints never make sense, so catch those too.

Make RenderObject.layout's isNormalized assert use the newer more fancy
debug version of isNormalized.

Upgrade command shouldn’t validate project, fixes #1745

Teach PageableList about scroll anchors

Teach ScrollableList about scroll anchors

fix an issue with logging ios simulator output

Introduce globals

Tapping through drag targets.

Factor out the HitTestBehavior logic so that RenderMetaData can use it.

Use that in DragTarget.

ignore derived resources in ios/build

Add a refresh command that rebuilds the snapshot and reloads it on th…

Add scroll anchor for Block

This patch teaches block how to anchor its scrolling to the end rather than the
start.

Fixes #136

Draggable: childWhenDragging, maxSimultaneousDrags

Fixes issue with replacing the root node

filter offline devices from notifications

rename the service protocol extensions

This provides a fast way to iterate on changes to a Flutter app that only
involve updates to Dart code and do not require a full build and install
of the FLX and APK

Remove FLUTTER_DEV environment var

This check is triggering even for folks who don't have FLUTTER_DEV set, which
is breaking their toolchain because we're not running `pub get` on the
flutter_tools package.

Update docs for benchmarks.

removed kStatusBarHeight constant