Report CompileTime metric in flutter build aot --report-timings.

[sortie]

This is the correct metric to report for compilation time benchmarks rather
than RunTime. Rename the 'gen_snapshot' value to merely 'snapshot' for
backwards compatibility and overall simplicity.

This change simplifies Dart's benchmarking of Flutter by making it easier to
adopt --report-timings (made for Dart to use), which makes the benchmarks
much more robust.

Description

This changes matches what the Dart benchmarking infrastructure needs a little better than the existing behavior. I don't believe anything else is using --report-timings, so this should be fine to slightly change.

Related Issues

The --report-timings flag was introduced in #30032 for use by dart's benchmarking infrastructure.

Tests

I updated the existing test to test the new behavior. I couldn't figure out how to run the test though, cd packages/flutter_tools $ flutter test test/base/build_test.dart failed to me with Failed to load test harness. Are you missing a dependency on flutter_test? and FileSystemException(uri=org-dartlang-untranslatable-uri:package%3Aflutter_test%2Fflutter_test.dart; message=StandardFileSystem only supports file:* and data:* URIs). Let's see if the PR checks work here on github.

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
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Breaking Change

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• Yes, this is a breaking change (Please read Handling breaking changes). Replace this with a link to the e-mail where you asked for input on this proposed change.
• No, this is not a breaking change.

[sortie]

@mraleph

[mraleph]

Merged for @sortie because he does not have repository access (AFAIK).

[sortie]

Thanks @mraleph, yeah I didn't. I'll take advantage of this CL then.