The compilation trace lists functions that will be compiled in the core
snapshot. It is derived by unioning the output of Dart_SaveCompilationTrace
after startup from a few apps.

To update this trace.
  - Replace the existing trace with an empty file.
  - In //topaz/runtime/flutter_runner/engine.cc, change kCompilationTraceDelayInSeconds to a non-zero value.
  - In //topaz/runtime/flutter_runner/meta/flutter_jit_runner.cmx, add "persistent-storage" to "features" section.
  - Build; run a few apps, collecting compilation trace from each app.
    - Look for "Dart compilation trace" in 'fx log' to make sure the trace is written.
    - Copy the compilation trace to the host: "fx cp --to-host /data/dart_compilation_trace.txt trace"
  - Process the trace(s) with merge_traces.dart to filter for whitelisted packages and sort.