Allow creating slimpeller engine variants. (flutter/engine#51824)

Fixes https://github.com/flutter/flutter/issues/144432

engine/src/flutter/common/config.gni

@@ -28,6 +28,13 @@ declare_args() {
   #TODO(cyanglaz): Remove above comment about test flag when the entire iOS embedder supports app extension
   #https://github.com/flutter/flutter/issues/124289
   darwin_extension_safe = false
+
+  # Whether binary size optimizations with the assumption that Impeller is the
+  # only supported rendering engine are enabled.
+  #
+  # See [go/slimpeller-dashboard](https://github.com/orgs/flutter/projects/21)
+  # for details.
+  slimpeller = false
 }
 
 # feature_defines_list ---------------------------------------------------------
@@ -62,6 +69,10 @@ if (flutter_runtime_mode == "debug") {
   feature_defines_list += [ "FLUTTER_RUNTIME_MODE=0" ]
 }
 
+if (slimpeller) {
+  feature_defines_list += [ "SLIMPELLER=1" ]
+}
+
 if (is_ios || is_mac) {
   flutter_cflags_objc = [
     "-Werror=overriding-method-mismatch",

engine/src/flutter/tools/gn

@@ -68,6 +68,9 @@ def get_out_dir(args):
   if args.darwin_extension_safe:
     target_dir.append('extension_safe')
 
+  if args.slimpeller:
+    target_dir.append('slimpeller')
+
   if args.target_dir != '':
     target_dir = [args.target_dir]
 
@@ -1260,6 +1263,15 @@ def parse_args(args):
       help='Do not run GN. Instead configure the Impeller cmake example build.',
   )
 
+  parser.add_argument(
+      '--slimpeller',
+      default=False,
+      action='store_true',
+      help='Enable optimizations that attempt to reduce binary size of the ' +
+      'Flutter engine by assuming only the Impeller rendering engine is supported.' +
+      'See [go/slimpeller-dashboard](https://github.com/orgs/flutter/projects/21) for details.'
+  )
+
   # Sanitizers.
   parser.add_argument('--asan', default=False, action='store_true')
   parser.add_argument('--lsan', default=False, action='store_true')