[vm/compiler] Fix build on ARM64 Windows.

Fixes the ARM64 Windows build error introduced by 9b06e26 due to
allocating an array of size 0 when the varargs argument to the
template is empty.

TEST=ci

Change-Id: Iccd714de5643da647db8de2a5bea9ef7074610c0
Cq-Include-Trybots: luci.dart.try:vm-win-release-arm64-try,vm-aot-win-release-arm64-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/398886
Auto-Submit: Tess Strickland <[email protected]>
Reviewed-by: Derek Xu <[email protected]>

Author: sstrickl

runtime/vm/compiler/backend/il_test_helper.cc

@@ -81,6 +81,19 @@ TypeParameterPtr GetFunctionTypeParameter(const Function& fun, intptr_t index) {
   return param.ptr();
 }
 
+ObjectPtr Invoke(const Library& lib, const char* name) {
+  Thread* thread = Thread::Current();
+  Dart_Handle api_lib = Api::NewHandle(thread, lib.ptr());
+  Dart_Handle result;
+  {
+    TransitionVMToNative transition(thread);
+    result =
+        Dart_Invoke(api_lib, NewString(name), /*argc=*/0, /*argv=*/nullptr);
+    EXPECT_VALID(result);
+  }
+  return Api::UnwrapHandle(result);
+}
+
 InstructionsPtr BuildInstructions(
     std::function<void(compiler::Assembler* assembler)> fun) {
   auto thread = Thread::Current();

runtime/vm/compiler/backend/il_test_helper.h

@@ -65,6 +65,8 @@ ClassPtr GetClass(const Library& lib, const char* name);
 TypeParameterPtr GetClassTypeParameter(const Class& klass, intptr_t index);
 TypeParameterPtr GetFunctionTypeParameter(const Function& fun, intptr_t index);
 
+ObjectPtr Invoke(const Library& lib, const char* name);
+
 InstructionsPtr BuildInstructions(
     std::function<void(compiler::Assembler* assembler)> fun);