To make the units clear.

A common pattern is to use a Positioned with a Sized box to give both an offset
from the edge as well as a fixed size to the child. This patch rolls into into
the Stack layout algorithm for simplicity.

Fixes #250

Prior to this patch, MultiChildLayoutDelegate couldn't be re-entered because it
cleared _idToChild when unwinding its stack. Now we restore the previous value
of _idToChild when we unwind.

Now we use a zero-width space to force the engine to resolve the font and tell
us how large the text is likely to be once there's text in the widget.

Fixes #302

Elaborate on the debugging information exposed by
RenderPointerListener, Listener, and GestureDetector.

- Drop the unused BindingHitTestEntry class. (Well, it was
  constructed, but its member was never used, so it wasn't doing
  anything a boring old HitTestEntry couldn't already do.)

- Add toString()s to HitTestEntry and HitTestResult, to aid in
  debugging hit-test related code.

This patch calls dispose() explicitly on a number of heavy C++ objects so that
they can be eagerly finalized instead of waiting for GC.

Other changes in this patch:
- Make the 'flutter' tool say "Updating flutter tool..." when it calls
  pub get, to avoid confusion about what the pub get output is about.
- Make the bash flutter tool call pub get when the revision has
  changed. (This was already happening on Windows.)
- Fix a raft of bugs found by the analyzer.
- Fix some style nits in various bits of code that happened to be near
  things the analyzer noticed.
- Remove the logic in "flutter test" that would run "pub get", since
  upon further reflexion it was determined it didn't work anyway.
  We'll probably have to add better diagnostics here and say to run the
  updater script.
- Remove the native velocity tracker script, since it was testing code
  that has since been removed.

Notes on ignored warnings:
- We ignore warnings in any packages that are not in the Flutter repo or
  in the author's current directory.
- We ignore various irrelevant Strong Mode warnings. We still enable
  strong mode because even though it's not really relevant to our needs,
  it does (more or less accidentally) catch a few things that are
  helpful to us.
- We allow CONSTANTS_LIKE_THIS, since we get some of those from other
  platforms that we are copying for sanity and consistency.
- We allow one-member abstract classes since we have a number of them
  where it's perfectly reasonable.
- We unfortunately still ignore warnings in mojom.dart autogenerated
  files. We should really fix those but that's a separate patch.
- We verify the actual source file when we see the 'Name non-constant
  identifiers using lowerCamelCase.' lint, to allow one-letter variables
  that use capital letters (e.g. for physics expressions) and to allow
  multiple-underscore variable names.
- We ignore all errors on lines that contain the following magic
  incantation and a "#" character:
    // analyzer doesn't like constructor tear-offs
- For all remaining errors, if the line contains a comment of the form
    // analyzer says "..."
  ...then we ignore any errors that have that "..." string in them.

The input widget scrolls, so it should give its text loose constraints.  That
way the text ends up being its intrinsic size even if put in a context with
tight constraints.

Fixes #298

Previously we would walk from every node in layout to the root to mark the root
as needing paint. Now we leave dirty bits throughout the tree so that we can
cut off the walk as soon as we find a dirty node.

Inside runApp, we were building the render tree for the app, but we weren't
calling layout, which meant we were running a hit test on a dirty tree. Now
hitTest specifically asserts that the tree is clean and we synchronously call
layout for your app during runApp.

Fixes #1960

This patch paves the path for refactoring the Drawer into the Scaffold to
support edge swipes and peeks.

A translucent gesture detector still listens for gestures but also lets the
content visually behind the detector receive events.

Now a RenderBox is considered hit if one of its children are hit or it itself
decides that it's hit. In particular, empty space inside a flex won't be hit
because none of the children are located there and a RenderFlex doesn't
consider itself hittable.

Fixes #53
Fixes #1221

- change how we expose settings at the RenderObject layer so that it's
  easier to maintain.
- expose the Widget owner chain in the RenderObject layer debug output
- add debug info to RenderOpacity, RenderIgnorePointer, RenderListener
- make the output for text nodes prettier

Previously, we'd leave the old values in the parent data if the types matches,
but not all render objects would reset these values during layout. For example,
RenderProxyBox doesn't set the position field because it doesn't read the
position field. However, leaving the old data there violates the invariants of
the box protocol and can cause trouble (e.g., localToGlobal giving the wrong
result).

Fixes #1939

CustomMultiChildLayout delegates to a MultiChildLayoutDelegate. Widget children are specified by their index in the list of children.

The Rendering layer binding now multiplexes the dart:ui metrics change
notification so that multiple consumers can all listen to the change.

A CustomOneChildLayout is a widget that lets you perform a custom layout in the
simplified setting of sizing and positioning a single child.

Previous these callbacks were leaking the implementation detail that they were
triggered by taps. In a later patch, we're going to add a parameter to
GestureTapCallback that these callbacks won't have.

Related to #1807

Instread of an explicit 'enabled' bool, this uses the presence of the
event handler to determine if a widget is enabled or not. This means
that if you've not passed a handler, your widget will be disabled, which
makes sense, since it wouldn't work anyway.

Adds this feature to checkbox, and ports raised button, flat button, and
radio buttons to this new model.

Adds a checkbox to card_collection that can be disabled.

Hide a (basically bogus) hint from the (soon to be disabled) strong hint
mode in the analyzer that this reveals.

We were assuming that the text extended from 0.0 to its max content width.
That's not correct for right-aligned text. Instead, we need to layout the text
again at the width we want it to occupy.

Fixes #1806

Implements a mode that highlights RenderBoxes while events are being
routed through them.

Also, moves the size painting to after paint, so that opaque boxes
don't obscure the debugging lines.

This makes skyanalyzer also check the examples, and fixes everything it
found there.

The magic for updating DebugSize's knowledge about whether it was
allowed to be used by the parent or not previously failed to correctly
deal with sizedByParent.

Fixes #1702