This updates the web_ui implementation of lerpDouble to match the
behaviour of the C++ engine implementation in dart:ui.

Specifically this covers the following changes:
* #20871: stricter handling of NaN and infinity
* #20879: Improve the precision of lerpDouble

lerpDouble: stricter handling of NaN and infinity (#20871)
----------------------------------------------------------

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.

Improve the precision of lerpDouble (#20879)
--------------------------------------------

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.

https://dart.googlesource.com/sdk.git/+log/b88c06c314f4..33b6c95936e0

2020-09-08 dart-luci-ci-builder@dart-ci.iam.gserviceaccount.com Version 2.10.0-103.0.dev
2020-09-08 dart-luci-ci-builder@dart-ci.iam.gserviceaccount.com Version 2.10.0-102.0.dev

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/dart-sdk-flutter-engine
Please CC dart-vm-team@google.com on the revert to ensure that a human
is aware of the problem.

To report a problem with the AutoRoller itself, please file a bug:
https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+doc/master/autoroll/README.md

Lazy async stacks were already enabled by-default in AOT mode in [0] - which made the
gen_snapshot invocations use "--lazy-async-stacks --no-causal-async-stacks".

This change does the same with the engine defaults, which makes this be enabled
by-default in JIT mode as well.

See go/dart-10x-faster-async for more information.

This is a re-land: A fix for what we believe to have caused the last revert has landed upstream in Dart in dart-lang/sdk@0004589

[0] flutter/flutter@3478232

* adjust blur radius for HTML to match CanvasKit

* switch to new goldens for new blur radius logic

[Android R] Sync keyboard animation with view insets vs Android 11/R/API 30 WindowInsetsAnimation (#20843)

dart-lang/sdk@016e8880f0a Version 2.10.0-90.0.dev
dart-lang/sdk@34f636ce1d8 Revert "[vm/io] Fix FileSystemWatcher enum name typo."
dart-lang/sdk@7a02118a410 [vm/fuchsia]: Safely shutdown the async message loop.
dart-lang/sdk@334d3681467 [analyzer_plugin] Remove unused dart:async imports
dart-lang/sdk@047c0f860a5 [Analyzer] Support analyzing open files without open workspaces
dart-lang/sdk@f846d0e7ea0 [analyzer_cli] Remove unused dart:async imports
dart-lang/sdk@ba372d6e8a4 [dartdev] Remove unused dart:async import
dart-lang/sdk@30cfb228c00 [nnbd_migration] Remove unused dart:async imports
dart-lang/sdk@1134a0f17c8 [analysis_server] Remove unused dart:async imports
dart-lang/sdk@3317d47c7de [_fe_analyzer_shared] Remove unused dart:async import
dart-lang/sdk@0a036db5962 [vm/io] Fix FileSystemWatcher enum name typo.
dart-lang/sdk@000f3135a5c [analyzer] Remove unused dart:async imports
dart-lang/sdk@7817aa17967 [dartfix] Remove unused dart:async imports
dart-lang/sdk@0144087619f Version 2.10.0-89.0.dev
dart-lang/sdk@42dfb9d3046 Add `generateCallGraphWithDominators` method to generate a `CallGraph` from a precompiler trace.
dart-lang/sdk@01024dfa5a8 Issue 43200. Report ASSIGNMENT_TO_FINAL when 'final late' has an initializer.
dart-lang/sdk@ab16d79af97 Revert "Flow analysis changes to fix mixed-mode unsoundness loophole."
dart-lang/sdk@7363adca67a [infra] Update checked-in SDKs to 2.10.0-79.0.dev
dart-lang/sdk@d833f2f65c1 Flow analysis changes to fix mixed-mode unsoundness loophole.
dart-lang/sdk@33e4a6bc2a8 Revert "[vm] Have Read/WriteStream use (S)LEB128 for variable-length encoding."
dart-lang/sdk@d9dc24b53e7 Version 2.10.0-88.0.dev
dart-lang/sdk@7130dd4d3d9 [cfe] Report error on non-void setter return type
dart-lang/sdk@a7cb764aa45 [tests/ffi] Reinstate deleted test
dart-lang/sdk@39689eb07cd [co19] Roll co19 to 9c07e26d67665de7972ba6f7e87369933a978545
dart-lang/sdk@1f92293fd58 Revert "[dart:html] Add 'creates' type description to JS calls"
dart-lang/sdk@e44cf969c64 Fix creates value for getStats
dart-lang/sdk@15201ebe5f1 Version 2.10.0-87.0.dev
dart-lang/sdk@eb8dfee1074 [CFE] Clean up duplicate entries in spell checker lists

Let FlutterActivity/Fragment/FragmentActivity have an app bundle path override instead of eager resolving during construction (#20769)