Move !is_android to build_engine_artifacts declaration (flutter#54006)

The `//flutter/build/archives:artifacts` target is used to build a zip
archive (artifacts.zip) of host tools such as flutter_tester, the Dart
kernel compiler, the impellerc shader compiler, and other tooling that
is bundled in debug-mode host builds.

This moves the `!is_android` to the definition of
`build_engine_artifacts`. This is required because of the way that we
produce 32-bit arm gen_snapshot for Android on Windows hosts, which
relies on the regular x64 host toolchain due to us having no 32-bit arm
toolchain for Windows. As such, `current_toolchain == host_toolchain` on
that platform.

```
build_engine_artifacts =
    flutter_build_engine_artifacts &&
    (current_toolchain == host_toolchain ||
     (is_linux && !is_chromeos && current_cpu != "arm") || is_mac || is_win)
```

On iOS builds, we don't have this issue since `current_toolchain` will
be one of:
* `//build/toolchain/mac:ios_clang_arm`
* `//build/toolchain/mac:ios_clang_arm_sim`
* `//build/toolchain/mac:ios_clang_x64_sim`

Whereas `host_toolchain` will be one of:
* `//build/toolchain/mac:clang_arm64`
* `//build/toolchain/mac:clang_x64`

This patch also adds documentation to clarify the purpose of this target
and where related artifacts are produced so that future readers don't
need to do a deep dive into our build plumbing to figure this out.

While the target itself is primarily intended for producing host
binaries, one target binary (gen_snapshot) is bundled into the same
archive bundle as the host tools. This should be refactored such that
just like iOS and Android, they are bundled into their own
target-platform-specific archive, and the tool code accordingly updated
to pull these down into the appropriate cache directory.

As a side-note, on macOS we do rely on this archive target for the host
tools, but the bundled gen_snapshot is unused -- instead, one produced
by the //flutter/sky/tools/create_macos_gen_snapshots.py script used.
This should be fixe in a followup patch.

Related: flutter/flutter#38935

Identified while trying to resolve:
Issue: flutter/flutter#101138
Issue: flutter/flutter#69157

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build/archives/BUILD.gn

@@ -21,7 +21,28 @@ generated_file("artifacts_entitlement_config") {
   }
 }
 
-if (build_engine_artifacts && !is_android) {
+# This target is used to build a zip archive (artifacts.zip) of host tools such
+# as flutter_tester, the Dart kernel compiler, the impellerc shader compiler,
+# and other tooling that is bundled in debug-mode host builds.
+#
+# For historical reasons, this bundle also includes gen_snapshot (which is a
+# target-platform-specific binary) for desktop platforms, though the
+# un-suffixed gen_snapshot is unused on macOS (gen_snapshot_arm64 and
+# gen_snapshot_x64 are used in its place).
+#
+# Target-specific iOS artifacts.zip are produced in:
+# //flutter/sky/tools/create_full_ios_framework.py
+#
+# Target-specific Android archives are produced in:
+# //flutter/shell/platform/android:gen_snapshot
+#
+# macOS-specific gen_snapshot_arm64 and gen_snapshot_x64 are produced in:
+# //flutter/sky/tools/create_macos_gen_snapshots.py
+#
+# TODO: https://github.com/flutter/flutter/issues/38935
+# Target-specific binaries such as gen_snapshot should be separated into a
+# separate archive target from host binaries such as flutter_tester.
+if (build_engine_artifacts) {
   zip_bundle("artifacts") {
     deps = [
       "$dart_src/runtime/bin:gen_snapshot",

common/config.gni

@@ -162,9 +162,15 @@ if (flutter_prebuilt_dart_sdk) {
 
 # Flutter SDK artifacts should only be built when either doing host builds, or
 # for cross-compiled desktop targets.
+#
+# !is_android is necessary because of the way that Android 32-bit arm
+# gen_snapshot is built on Windows using the host toolchain, since we have no
+# 32-bit arm toolchain for Windows. See comments in //flutter/tools/gn for
+# details.
+#
 # TODO: We can't build the engine artifacts for arm (32-bit) right now;
-# see https://github.com/flutter/flutter/issues/74322
+# see https://github.com/flutter/flutter/issues/74322.
 build_engine_artifacts =
-    flutter_build_engine_artifacts &&
+    flutter_build_engine_artifacts && !is_android &&
     (current_toolchain == host_toolchain ||
      (is_linux && !is_chromeos && current_cpu != "arm") || is_mac || is_win)