Compile client code with dart2wasm (#6953)

This configures `build_web_compilers` to use ddc on dev builds
(`dash_site serve`) while compiling with both `dart2js` and `dart2wasm`
on release modes (`dash_site build`, or the added `--release` flag to
`serve` to experiment with this more easily).

Using jaspr CLI options to switch to dart2wasm would _only_ compile
sources with `dart2wasm`, this uses both compilers and a feature
detection snippet emitted by `dart2wasm` to determine which variant is
used. That seems to be in spirit of
#6854.

To stop jaspr from overriding options passed to `build_web_compilers`,
one needs to use the `--no-managed-build-options` flag. A downside is
that the `PRODUCTION` define set on release modes would not be forwarded
to client code any longer. The constant is only used in the `robots.txt`
generator so this should be fine, forwarding this to client code would
require more changes to `jaspr_cli`.

Closes #6854.

Author: simolus3

site/build.yaml

@@ -7,3 +7,21 @@ builders:
         - "lib/src/style_hash.dart"
     auto_apply: root_package
     build_to: source
+
+targets:
+  $default:
+    builders:
+      build_web_compilers:entrypoint:
+        dev_options:
+          compilers:
+            dartdevc:
+        release_options:
+          # Use production compilers for release builds, and ensure
+          # kReleaseMode in jaspr is enabled.
+          compilers:
+            dart2js:
+              args: [-O4, -Djaspr.flags.release=true]
+            dart2wasm:
+              # Use -O2 to catch more potential runtime issues, it is not much
+              # slower than -O4: https://github.com/dart-lang/site-www/pull/6953#discussion_r2453318668
+              args: [-O2, -Djaspr.flags.release=true]

tool/dash_site/lib/src/commands/build.dart

@@ -37,6 +37,9 @@ final class BuildSiteCommand extends Command<int> {
         'run',
         'jaspr_cli:jaspr',
         'build',
+        // Use build_web_compiler options specified in build.yaml instead of
+        // those specified by jaspr_cli.
+        '--no-managed-build-options',
         '--sitemap-domain=https://dart.dev',
         '--dart-define=PRODUCTION=$productionRelease',
       ],

tool/dash_site/lib/src/commands/serve.dart

@@ -11,8 +11,17 @@ final class ServeSiteCommand extends Command<int> {
   @override
   String get name => 'serve';
 
+  ServeSiteCommand() {
+    argParser.addFlag(
+      'release',
+      defaultsTo: false,
+      help: 'Build with compilers used for release builds instead of dartdevc.',
+    );
+  }
+
   @override
   Future<int> run() async {
+    final release = argResults!.flag('release');
     installJasprCliIfNecessary();
 
     final process = await Process.start(
@@ -23,7 +32,11 @@ final class ServeSiteCommand extends Command<int> {
         'run',
         'jaspr_cli:jaspr',
         'serve',
+        // Use build_web_compiler options specified in build.yaml instead of
+        // those specified by jaspr_cli.
+        '--no-managed-build-options',
         '--dart-define=PRODUCTION=false',
+        if (release) '--release',
       ],
       workingDirectory: 'site',
       runInShell: true,