--- a/js/src/shell/js.cpp
+++ b/js/src/shell/js.cpp
@@ -309,16 +309,17 @@ static bool enableDisassemblyDumps = fal
 static bool offthreadCompilation = false;
 static bool enableBaseline = false;
 static bool enableIon = false;
 static bool enableAsmJS = false;
 static bool enableNativeRegExp = false;
 static bool enableUnboxedArrays = false;
 static bool enableSharedMemory = SHARED_MEMORY_DEFAULT;
 static bool enableWasmAlwaysBaseline = false;
+static bool enableAsyncStacks = false;
 #ifdef JS_GC_ZEAL
 static uint32_t gZealBits = 0;
 static uint32_t gZealFrequency = 0;
 #endif
 static bool printTiming = false;
 static const char* jsCacheDir = nullptr;
 static const char* jsCacheAsmJSPath = nullptr;
 static RCFile* gErrFile = nullptr;
@@ -7127,24 +7128,26 @@ static bool
 SetContextOptions(JSContext* cx, const OptionParser& op)
 {
     enableBaseline = !op.getBoolOption("no-baseline");
     enableIon = !op.getBoolOption("no-ion");
     enableAsmJS = !op.getBoolOption("no-asmjs");
     enableNativeRegExp = !op.getBoolOption("no-native-regexp");
     enableUnboxedArrays = op.getBoolOption("unboxed-arrays");
     enableWasmAlwaysBaseline = op.getBoolOption("wasm-always-baseline");
+    enableAsyncStacks = op.getBoolOption("async-stacks");
 
     JS::ContextOptionsRef(cx).setBaseline(enableBaseline)
                              .setIon(enableIon)
                              .setAsmJS(enableAsmJS)
                              .setWasm(true)
                              .setWasmAlwaysBaseline(enableWasmAlwaysBaseline)
                              .setNativeRegExp(enableNativeRegExp)
-                             .setUnboxedArrays(enableUnboxedArrays);
+                             .setUnboxedArrays(enableUnboxedArrays)
+                             .setAsyncStack(enableAsyncStacks);
 
     if (op.getBoolOption("no-unboxed-objects"))
         jit::JitOptions.disableUnboxedObjects = true;
 
     if (const char* str = op.getStringOption("cache-ir-stubs")) {
         if (strcmp(str, "on") == 0)
             jit::JitOptions.disableCacheIR = false;
         else if (strcmp(str, "off") == 0)
@@ -7593,16 +7596,17 @@ main(int argc, char** argv, char** envp)
                             "(default: # of cores - 1)", -1)
         || !op.addBoolOption('\0', "ion", "Enable IonMonkey (default)")
         || !op.addBoolOption('\0', "no-ion", "Disable IonMonkey")
         || !op.addBoolOption('\0', "no-asmjs", "Disable asm.js compilation")
         || !op.addBoolOption('\0', "no-native-regexp", "Disable native regexp compilation")
         || !op.addBoolOption('\0', "no-unboxed-objects", "Disable creating unboxed plain objects")
         || !op.addBoolOption('\0', "unboxed-arrays", "Allow creating unboxed arrays")
         || !op.addBoolOption('\0', "wasm-always-baseline", "Enable experimental Wasm baseline compiler when possible")
+        || !op.addBoolOption('\0', "async-stacks", "Enable capturing of async stacks, e.g. for Promises")
 #ifdef ENABLE_SHARED_ARRAY_BUFFER
         || !op.addStringOption('\0', "shared-memory", "on/off",
                                "SharedArrayBuffer and Atomics "
 #  if SHARED_MEMORY_DEFAULT
                                "(default: on, off to disable)"
 #  else
                                "(default: off, on to enable)"
 #  endif