Add --trace flag to flutter build apk for build profiling

[eseidel]

Summary

• Adds a --trace option to flutter build apk, flutter build ios, and flutter build ipa that produces a Chrome Trace Event Format JSON file
• The trace shows where time is spent across three layers: flutter tool overhead, native build system (Gradle/Xcode), and flutter assemble targets
• Output viewable in Perfetto at https://ui.perfetto.dev

Usage

flutter build apk --trace=build_trace.json
flutter build ios --trace=build_trace.json
flutter build ipa --trace=build_trace.json

Implementation

Core:

• New BuildTracer utility class for collecting, merging, and writing Chrome Trace Event Format events
• --trace-file option threaded through both Gradle plugin (Kotlin) and Xcode build phases (xcode_backend.dart) to flutter assemble
• Wall-clock timestamps added to PerformanceMeasurement for accurate cross-process timing

Android path: gradle.dart → Gradle -Ptrace-file property → FlutterPlugin.kt → BaseFlutterTaskHelper.kt → flutter assemble --trace-file

iOS path: mac.dart → TRACE_FILE Xcode build setting → xcode_backend.dart → flutter assemble --trace-file

Three trace rows (tids) in Perfetto:

1. flutter tool (tid 1): pre-build setup, post-build processing, overall build
2. native build system (tid 2): Gradle or Xcode subprocess
3. flutter assemble (tid 3): individual build targets (KernelSnapshot, AotElfRelease/AotAssembly, CopyAssets, etc.)

Test plan

• Unit tests for BuildTracer and BuildTraceEvent (8 tests)
• Unit test for writeTraceData in assemble
• Existing assemble tests pass (19 tests)
• Existing build_system tests pass (39 tests)
• Existing build_info tests pass (18 tests)
• dart analyze clean on all modified files
• Manual: flutter build apk --trace=trace.json on a sample app, view in Perfetto
• Manual: flutter build ios --trace=trace.json on a sample app, view in Perfetto