Be explicit about the sysroot in the toolchain definition instead of inferring the same from active arguments.

engine/src/flutter/build/config/ios/ios_sdk.gni

@@ -18,18 +18,31 @@ declare_args() {
 
   # The iOS Code signing identity to use
   ios_code_signing_identity = ""
+
+  # The path to the iOS device SDK.
+  ios_device_sdk_path = ""
+
+  # The path to the iOS simulator SDK.
+  ios_simulator_sdk_path = ""
 }
 
-if (ios_sdk_path == "") {
-  # Compute default target.
-  if (use_ios_simulator) {
-    _ios_sdk_to_query = "iphonesimulator"
-  } else {
-    _ios_sdk_to_query = "iphoneos"
-  }
-  _ios_sdk_result =
-      exec_script("ios_sdk.py", [ _ios_sdk_to_query ], "list lines")
-  ios_sdk_path = _ios_sdk_result[0]
+if (ios_device_sdk_path == "") {
+  _ios_device_sdk_result =
+      exec_script("ios_sdk.py", [ "iphoneos" ], "list lines")
+  ios_device_sdk_path = _ios_device_sdk_result[0]
+}
+
+if (ios_simulator_sdk_path == "") {
+  _ios_sim_sdk_result =
+      exec_script("ios_sdk.py", [ "iphonesimulator" ], "list lines")
+  ios_simulator_sdk_path = _ios_sim_sdk_result[0]
+}
+
+# Compute default target.
+if (use_ios_simulator) {
+  ios_sdk_path = ios_simulator_sdk_path
+} else {
+  ios_sdk_path = ios_device_sdk_path
 }
 
 if (use_ios_simulator) {

engine/src/flutter/build/config/mac/BUILD.gn

@@ -6,9 +6,6 @@ import("//build/config/sysroot.gni")
 
 config("sdk") {
   common_flags = [
-    "-isysroot",
-    sysroot,
-    "-mmacosx-version-min=$mac_sdk_min",
     "-stdlib=libc++",
   ]
 

engine/src/flutter/build/toolchain/mac/BUILD.gn

@@ -8,6 +8,7 @@
 
 import("../goma.gni")
 import("//build/config/ios/ios_sdk.gni")
+import("//build/config/mac/mac_sdk.gni")
 
 assert(host_os == "mac")
 
@@ -217,7 +218,7 @@ mac_toolchain("ios_clang_arm") {
   cxx = "${goma_prefix}$prefix/clang++"
   ld = cxx
   is_clang = true
-  sysroot_flags = "-isysroot $sysroot -miphoneos-version-min=$ios_deployment_target"
+  sysroot_flags = "-isysroot $ios_device_sdk_path -miphoneos-version-min=$ios_deployment_target"
 }
 
 # Toolchain used for iOS simulator targets.
@@ -230,7 +231,7 @@ mac_toolchain("ios_clang_x64") {
   cxx = "${goma_prefix}$prefix/clang++"
   ld = cxx
   is_clang = true
-  sysroot_flags = "-isysroot $sysroot -mios-simulator-version-min=$ios_deployment_target"
+  sysroot_flags = "-isysroot $ios_simulator_sdk_path -mios-simulator-version-min=$ios_deployment_target"
 }
 
 # Toolchain used for Mac host targets.
@@ -243,6 +244,7 @@ mac_toolchain("clang_x64") {
   cxx = "${goma_prefix}$prefix/clang++"
   ld = cxx
   is_clang = true
+  sysroot_flags = "-isysroot $mac_sdk_path -mmacosx-version-min=$mac_sdk_min"
 }
 
 # Toolchain used for Mac host (i386) targets.
@@ -255,4 +257,5 @@ mac_toolchain("clang_i386") {
   cxx = "${goma_prefix}$prefix/clang++"
   ld = cxx
   is_clang = true
+  sysroot_flags = "-isysroot $mac_sdk_path -mmacosx-version-min=$mac_sdk_min"
 }