Fixes https://github.com/dart-lang/sdk/issues/61684
This caching is based largely on the `--depfile` feature offered by
`dart compile`, which is based on a Ninja depfile concept
(https://ninja-build.org/manual.html#_depfile), which spits out a file
(`depfile.txt` here) which lists all of the input files which were
required to build an AOT snapshot.
The process is essentially:
1. If an AOT snapshot is found, maybe use it as a cached snapshot!
a. If the `pubspec.yaml` modification timestamp is newer, re-compile!
b. If the `.dart_tool/package_config.json` modification timestamp is
newer, re-compile!
c. If the `bin/plugin.dart` modification timestamp is newer,
re-compile!
d. If the `bin/depfile.txt` file is missing or malformed, re-compile!
e. If any files mentioned in `bin/depfile.txt` have a newer
modification timestamp, or don't exist, or are an otherwise bad
path, re-compile!
f. Otherwise, save a dozen seconds and use the cached snapshot.
Change-Id: Icc747198f8af76d256ac915685473d6f529a3cef
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/464602
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Sam Rawlins
29b942c3f0