Reduces errors caused by the loss of floating point precision when the
two extrema of the lerp differ significantly in magnitude. Previously,
we used the calculation:

    a + (b - a) * t

When the difference in magnitude between `a` and `b` exceeds the
precision representable by double-precision floating point math, `b - a`
results in the larger-magnitude value of `a` or `b`. The error between
the value produced and the correct value is then scaled by t.

A simple example of the impact can be seen when `a` is significantly
larger in magnitude than `b`. In that case, `b - a` results in `a` and
when `t` is 1.0, the resulting value is `a - (a) * 1.0 == 0`.

The patch transforms the computation to the mathematically-equivalent
expression:

    a * (1.0 - t) + b * t

By scaling each value independently, the behaviour is more accurate.
From the point of view of performance, this adds an extra
multiplication, but multiplication is relatively cheap and the behaviour
is significantly better.

This patch also adds a `precisionErrorTolerance` constant to
test_utils.dart and migrates existing tests to use `closeTo()` for
testing.

The tests themselves *do* currently use values that have an exact
floating-point representation, but we should allow for flexibility in
future implementation changes.

Previously, the behaviour of lerpDouble with respect to NaN and infinity
was relatively complex and difficult to reason about. This patch
simplifies the behaviour with respect to those conditions and adds
documentation and tests.

In general, if `a == b` or both values are null, infinite, or NaN, `a`
is returned. Otherwise we require `a` and `b` and `t` to be finite or
null and the result of the linear interpolation is returned.

The behaviour of lerpDouble with respect to null inputs isn't entirely
obvious. In the case where both inputs are null, it returns null.
Otherwise, it defaults the null parameter to 0.0 and carries on.

Post non-null by default, it might be nice to strengthen the parameter
contract to require them to be non-null. While this would be a breaking
change, it seems likely that the framework either meets this guarantee
or can provide it without a framework breaking change.

https://github.com/flutter/flutter/issues/64617 tracks the above.

In the meantime, adding a test to lock in the current behaviour.

* Manual merge or Dart into flutter a3815b6590...24c7666def

dart-lang/sdk@24c7666def Update tests that have the current version number hardcoded
dart-lang/sdk@f88ce7aef5 Increase Dart version to 2.10

* Update language version.

* Update license.

* Hpdate language version.

* Update license.

* apply null safety syntax to mobile dart:ui

* nullability clean-ups

* opt out dart:ui from nnbd

* Annotate tests as well

* Analysis cleanup of the Dart in the Engine repo

* Update painting.dart

* Update painting.dart

This name is consistent with how the other `dart:` libraries are named now.

Fixes https://github.com/flutter/flutter/issues/5783

This change disentangles the build for the snapshotter from the main engine
build. Also, we now have all the Dart files for dart:ui in the same directory.

This patch starts down the process of cleaning up the dart:ui API. In this
approach, we use Dart code to specify the interface instead of IDL. The code in
this patch is far from complete. It's just a sketch to see how this sort of
approach might work.