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.

Enable ALL THE LINTS

Well, all the easy ones, anyway.

For some reason `// ignore:` isn't working for me so I've disabled
lints that need that. Also disabled those that require a ton of work
(which I'm doing, but not in this PR, to keep it reviewable).

This adds:
- avoid_init_to_null
- library_names
- package_api_docs
- package_names
- package_prefixed_library_names
- prefer_is_not_empty
- sort_constructors_first
- sort_unnamed_constructors_first
- unnecessary_getters_setters

Remove the _EquationMember.hashCode override

Fix crash when dumping the app if it uses RichText

Specifically:

* Handle null styles in TextSpan without crashing in toString().

* Handle null children in TextSpan child lists without crashing in
  toString().

* Handle entirely empty TextSpans in toString() explicitly.

* Assert that TextSpans don't contain nulls in various places. This is
  done more often than one might think necessary, because it turns out
  that TextSpan takes a (mutable) List for one of its arguments, so
  who knows what it will contain at any given time. By asserting all
  over the place, hopefully we'll catch it near the change if they do
  change it.

* Add a RichText example to Stocks to exercise RichText and TextSpans.

See also: https://github.com/flutter/flutter/issues/2514, https://github.com/flutter/flutter/issues/2519

Prepare to make RenderObjectElement buildable

This patch prepares us to pass a BuildContext to RenderObjectWidgets, which
will make it possible to rebuild RenderObjectElements:

 * Delay creation of the render object until mount(). That will let us pass
   `this` to createRenderObject and have the inherited elements be initialized.

 * Cleanup widgets that take builder closures to prepare for their
   RenderObjectElement to be rebuilt more often.

 * Add a test for the interaction between inherited widgets and
   MixedViewport.

Related to #2598

Use a member function instead of an anonymous closure

Since we removed our operator== overload, we can now allow people to put
these in collections again.

Also, turn on two more analyzer lints: avoid_empty_else and hash_and_equals.

Update engine revision to pick up fixes for precompilation variants

It's slightly more efficient.

Use the .. pattern more in updateRenderObject

Initial support for links in markdown

move the analysis_options to a file

Revert "Add DevicePortForwarder"

[ios] reduce xcode version requirement to 7.0

Update engine

Add DevicePortForwarder

Build bots only have 7.0.

Add an option to specify a working directory for the FLX builder

[ios] look for the best available devicetype/runtime when booting

Add a basic Text Fields demo to Material Gallery app.

BUG=https://github.com/flutter/flutter/issues/1547

`flutter run` should fail if `pub get` fails.

Previously we were ignoring the return code and continuing.

@devoncarew

Scaffold saves and restores scrollable state, etc

Update engine

Fixes #2569

Enable always_declare_return_types lint