Revert "AsyncSnapshot.data to throw if error or no data"

[tvolkert]

Reverts #34626

Change needs more discussion, so I'm stopping it from going to dev until the discussion is resolved.

[codecov]

Codecov Report

Merging #36618 into master will increase coverage by 1.18%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #36618      +/-   ##
==========================================
+ Coverage   54.55%   55.74%   +1.18%     
==========================================
  Files         188      188              
  Lines       17432    17421      -11     
==========================================
+ Hits         9510     9711     +201     
+ Misses       7922     7710     -212

Flag	Coverage Δ
#flutter_tool	55.74% <ø> (+1.18%)	⬆️

Impacted Files	Coverage Δ
packages/flutter_tools/lib/src/cache.dart	44.21% <0%> (-14.87%)	⬇️
packages/flutter_tools/lib/src/version.dart	95.07% <0%> (-2.47%)	⬇️
...ckages/flutter_tools/lib/src/ios/code_signing.dart	96.96% <0%> (-1.52%)	⬇️
packages/flutter_tools/lib/src/compile.dart	74.75% <0%> (-0.34%)	⬇️
.../flutter_tools/lib/src/android/android_device.dart	33.5% <0%> (-0.26%)	⬇️
packages/flutter_tools/lib/src/vmservice.dart	38.97% <0%> (-0.16%)	⬇️
.../flutter_tools/lib/src/fuchsia/fuchsia_device.dart	55.76% <0%> (+0.37%)	⬆️
packages/flutter_tools/lib/src/base/utils.dart	94.5% <0%> (+1%)	⬆️
packages/flutter_tools/lib/src/asset.dart	86.55% <0%> (+4.62%)	⬆️
packages/flutter_tools/lib/src/run_hot.dart	48.68% <0%> (+14.91%)	⬆️
... and 2 more

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[collinjackson]

lgtm