Many of the widgets that use CustomPaint were spamming repaints because
CustomPaint repaints when the identity of the onPaint callback changes, which
it does every build for StatelessComponents.

This patch changes CustomPaint to use a CustomPainter, similar to the new
custom layout widgets. The CustomPainter has a `shouldRepaint` function along
with its `paint` function. This function gives clients explicit control over
when the custom paint object repaints.

Shadows now render as three seprate MaskFilter.blur components per the most recent Material spec.

The shadows Map was replaced by a similar Map called elevationToShadow with entries that match the 10 elevations specifed by http://www.google.com/design/spec/what-is-material/elevation-shadows.html.

The "level" property (many classes) is now called "elevation", to match the Material spec.

BoxShadow now includes a spreadRadius parameter - as in CSS box-shadow. Renamed the BoxShadow blur property to blurRadius to further align BoxShadow with CSS box-shadow.

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.

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

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

Also, give RenderViewport a default offset.

I accidentally lost this when extracting RenderOverflowBox from
RenderProxyBox.

- Rename EdgeDims constructor to EdgeDims.TRBL().

- Add operator== to Size and Offset so that you can compare Size to
  DebugSize in checked mode.

- Add Size.lerp().

- Add various operators to EdgeDims. (*, /, ~/, %)

- Add EdgeDims.lerp().

- Update style guide. I went there to fix an EdgeDims constructor
  example, and stayed because some recent things came up and I wanted to
  add them before I forgot.

Also, introduce Colors and Typography to hold the material colors and the
typography declarations. Previously we expected clients of these libraries to
import them into a namespace, but that doesn't play nice with re-exporting them
from material.dart.

RenderView has to be a singleton for sanity during tests, otherwise they
all end up in the dirty lists and we end up pumping all of them each frame.

Almost done adding dartdoc to the rendering layer.

Turns out many of the functions on BoxConstraints weren't used or had callers
that could easily be updated to other functions. I've added dartdoc to all the
public functions as well as renamed some functions that had similar names but
did different things.

This patch makes Center and Align expand by default, which is usually what you
want. It also adds a ShrinkWrap option to let you shrink wrap in one or both
directions if that's really what you want to do.

Rather than reading out the maxWidth, we should call constrainWidth to factor
in the minWidth, which might be bigger.

@abarth

Also, rename build_utils.dart to widget_tester.dart. These files are now named
for their most commonly used classes.

Finally, add a .analysis_options to silence the (intentional) analyzer warnings
in append_child_test.dart.

We can make all the test pass by removing more of the mocks and using the real
code.

- add debugDescribeSettings to a few classes that were missing it
- fix some minor bugs in RenderShrinkWrapWidth and ShrinkWrapWidth
- introduce RenderShrinkWrapHeight and ShrinkWrapHeight

We need to implement visitChildren for the RenderObjects in the sector demo.
Also, add a test.

Fixes #790

Also, I've organized the tests by the render object they're testing.