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

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

And fix a zillion omissions this uncovered.

[engine] bump version to f2d8d75897a5aadcd9be0067a78fce1cea677fd6

Make flutter analyze useful for package conflicts

Also, resolve our package conflict, since reflectable has been fixed.

Not as