Report CompileTime metric in flutter build aot --report-timings. (#31895)

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.

packages/flutter_tools/lib/src/base/build.dart

@@ -203,7 +203,7 @@ class AOTSnapshotter {
     // }
 
     final SnapshotType snapshotType = SnapshotType(platform, buildMode);
-    final int genSnapshotExitCode = await _timedStep('gen_snapshot', () => genSnapshot.run(
+    final int genSnapshotExitCode = await _timedStep('snapshot(CompileTime)', () => genSnapshot.run(
       snapshotType: snapshotType,
       additionalArgs: genSnapshotArgs,
       iosArch: iosArch,
@@ -315,7 +315,7 @@ class AOTSnapshotter {
 
     final String depfilePath = fs.path.join(outputPath, 'kernel_compile.d');
     final KernelCompiler kernelCompiler = await kernelCompilerFactory.create(flutterProject);
-    final CompilerOutput compilerOutput = await _timedStep('frontend', () => kernelCompiler.compile(
+    final CompilerOutput compilerOutput = await _timedStep('frontend(CompileTime)', () => kernelCompiler.compile(
       sdkRoot: artifacts.getArtifactPath(Artifact.flutterPatchedSdkPath, mode: buildMode),
       mainPath: mainPath,
       packagesPath: packagesPath,
@@ -361,7 +361,7 @@ class AOTSnapshotter {
     final Stopwatch sw = Stopwatch()..start();
     final T value = await action();
     if (reportTimings) {
-      printStatus('$marker(RunTime): ${sw.elapsedMilliseconds} ms.');
+      printStatus('$marker: ${sw.elapsedMilliseconds} ms.');
     }
     return value;
   }

packages/flutter_tools/test/base/build_test.dart

@@ -525,7 +525,7 @@ void main() {
 
       expect(genSnapshotExitCode, 0);
       expect(genSnapshot.callCount, 1);
-      expect(bufferLogger.statusText, matches(RegExp(r'gen_snapshot\(RunTime\): \d+ ms.')));
+      expect(bufferLogger.statusText, matches(RegExp(r'snapshot\(CompileTime\): \d+ ms.')));
     }, overrides: contextOverrides);
   });