Introduce fast path for buffered ReadableStream (#13704)

Co-authored-by: Jarred-Sumner <Jarred-Sumner@users.noreply.github.com>

src/codegen/buildTypeFlag.ts

@@ -0,0 +1,16 @@
+const buildTypeFlag = process.argv.find(argv => {
+  if (argv.startsWith("--build-type=")) {
+    return argv;
+  }
+});
+
+const enum BuildType {
+  debug,
+  release,
+}
+if (buildTypeFlag) {
+  process.argv.splice(process.argv.indexOf(buildTypeFlag), 1);
+}
+let buildType = buildTypeFlag ? BuildType[buildTypeFlag.split("=")[1].toLowerCase()] : BuildType.release;
+
+export { BuildType, buildType };