[build] Add a build flag for disabling secure sockets in the Dart IO library

See flutter/flutter#170459

TEST=local builds of Flutter

Bug: flutter/flutter#170459
Change-Id: Id15b9e19000bde41ae67274e5d2e43c0a0ae7a85
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/434984
Reviewed-by: Siva Annamalai <[email protected]>
Commit-Queue: Jason Simmons <[email protected]>

Author: jason-simmons

runtime/bin/BUILD.gn

@@ -535,6 +535,10 @@ template("dart_io") {
       }
     }
 
+    if (dart_disable_secure_socket) {
+      defines += [ "DART_IO_SECURE_SOCKET_DISABLED" ]
+    }
+
     include_dirs = [
       "..",
       "//third_party",
@@ -845,6 +849,9 @@ template("dart_executable") {
     if (exclude_kernel_service) {
       defines += [ "EXCLUDE_CFE_AND_KERNEL_PLATFORM" ]
     }
+    if (dart_disable_secure_socket) {
+      defines += [ "DART_IO_SECURE_SOCKET_DISABLED" ]
+    }
     include_dirs = [
       "..",
       "//third_party",

runtime/runtime_args.gni

@@ -48,6 +48,9 @@ declare_args() {
   # verified at the operating system level.
   dart_use_fallback_root_certificates = false
 
+  # Whether to disable support for secure sockets in the Dart IO library.
+  dart_disable_secure_socket = false
+
   # Whether to link Crashpad library for crash handling. Only supported on
   # Windows for now.
   dart_use_crashpad = false