Based on feedback from the material design team.

Fixes #1613

These height values are incorrect

They're actually leading values rather than line height values, which means
they're operative between paragraphs not within a given paragraph.

fix image loading

Improve docs for TextStyle#height

Fixes #2918

Update engine

rev mockito; remove deprecated uses

Also, pass font information to ui.ParagraphStyle.

run pub with --no-package-symlinks

The intrinsic size checks were very slow

Now we run them once per render object.

Fixes #2487

Reactivating a StatefulElement should imply a build

Fixes #2901

More table fixes

Animate TwoLevelList leading icon color

Add keys to ResourceImages.

[driver] fix observatory port # and timeline data extraction

The way we pick observatory port # has changed and we have broken
logic that handles port 8181. To fix the buildbot, switch to port
8182. We can later figure out what we want to do when we clean up
port handling.

The old VM extention for extracting the timeline data is gone.
Switch to the new '_getVMTimeline' API.

Use _ScaffoldLayout to position scroll-resizable appbars

Table widget

- Add x and y to the parent data, in case it's useful. In point of fact,
  I ended up not needing it.

- Make columnWidths settable rather than having a setter so that the
  code in the widgets layer is more idiomatic.

- Teach setFlatChildren about avoiding unnecessary drop/adopt loops.

- Assert that the child list length is correct more aggressively so that
  we catch the dumb bug I had more quickly next time.

- Improve the toString() to handle empty tables better.

- Resort the imports in framework.dart.

- Introduce a LocalKey for cases where you don't want to allow use of
  GlobalKey.

- Make inflateWidget and deactivateChild public for subclass use.

- Table widget.

- Cell widget.

- TableRow fake widget.

Allow Draggables to drop through DragTargets that don't accept them to one that does.

adjust upgrade command

Fix fullscreen dialog gallery example

More elaborate exceptions.

AnimatedOpacity

This provides an easy way to do implicit opacity transitions.

The elaboration will continue until morale improves.

Specific exceptions considered here:

 - size setter checking when you set the size
 - layout verifying that you do set the size
 - hitTest verifying that you aren't dirty and have a size
 - flex complaining about canFlex

Adds constants for FractionalOffsets