Remove several entries from invocation fuzzer blacklist.

runtime/lib/object.cc

Index: runtime/lib/object.cc
diff --git a/runtime/lib/object.cc b/runtime/lib/object.cc
index 701fccd44ac9144524f57c2292c185c7b417f842..278cb0935d0f4e82903db64ea945c9d9c596f7e1 100644
--- a/runtime/lib/object.cc
+++ b/runtime/lib/object.cc
@@ -46,7 +46,9 @@ DEFINE_NATIVE_ENTRY(Object_setHash, 2) {
 
 DEFINE_NATIVE_ENTRY(Object_toString, 1) {
   const Instance& instance = Instance::CheckedHandle(arguments->NativeArgAt(0));
-  ASSERT(!instance.IsString());  // See issue 20583.
+  if (instance.IsString()) {
+    return instance.raw();
+  }
   const char* c_str = instance.ToCString();
   return String::New(c_str);
 }