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Fixes flutter/flutter#60628

* use Object for operator==

* apply style guide for operator==

This set of tests is already running on LUCI.

Bug:
  https://github.com/flutter/flutter/issues/60607

Fixes https://github.com/flutter/flutter/issues/59109

https://dart.googlesource.com/sdk.git/+log/f91547d6dd45..019755f77bdc

2020-06-30 paulberry@google.com Flow analysis: simplify marking of loop-assigned variables as not-unassigned
2020-06-30 regis@google.com Roll benchmarks-internal to 2d9d76e04ce0a997ad3eb4e5e320e89e727af8a5
2020-06-29 brianwilkerson@google.com Add a fix for prefer_final_in_for_each (issue 42510)
2020-06-29 brianwilkerson@google.com Unify two error codes for diagnostics
2020-06-29 scheglov@google.com Move OperationPerformance out of Cider to use it wider.
2020-06-29 nshahan@google.com [ddc/dart2js] Add failure case when setting list length
2020-06-29 zichangguo@google.com Update Duration.toString() description
2020-06-29 scheglov@google.com Test UNNECESSARY_CAST for FunctionType(s) and NullSafety.
2020-06-29 scheglov@google.com Stop using AnalysisOptions.signature and multiple paths in SdkDescription.
2020-06-29 scheglov@google.com Make all parameters of ExperimentalFeature constructor named required.
2020-06-29 danny@tuppeny.com Rename "Sort Directives" as "Organize Imports" in the quick-fix
2020-06-29 askesc@google.com [vm] Adjust internal UTF-8 encoder to match dart:convert.
2020-06-29 cskau@google.com [SDK] Clean up tools/utils.py. Order consts, helpers.
2020-06-29 lrn@google.com Add `Stream.multi` constructor.
2020-06-29 mit@google.com Roll stagehand package
2020-06-29 whesse@google.com Revert "[build] Use Goma from depot_tools instead of $HOME/goma"
2020-06-28 scheglov@google.com Fix for crash when computing the context type and unresolved named argument.
2020-06-28 brianwilkerson@google.com Improve the range selected by const_constructor_with_field_initialized_by_non_const
2020-06-28 brianwilkerson@google.com Do not report a missing implicit default super constructor when subclassing an non-subclassable class
2020-06-27 paulberry@google.com Migration: revert informative region height changes.

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

We can build unoptimized code for easier debugging with this flag.

Re-attempt at #19162 which seems to be stuck on a bad framework revision