Handle Bigint.toCString when remainder equals 0.

runtime/vm/bigint_operations_test.cc

Index: runtime/vm/bigint_operations_test.cc
diff --git a/runtime/vm/bigint_operations_test.cc b/runtime/vm/bigint_operations_test.cc
index 398339f9944157365f90b46c9a493e68df677712..75b24642ef4da04f1d98402b9838e202496f0a3f 100644
--- a/runtime/vm/bigint_operations_test.cc
+++ b/runtime/vm/bigint_operations_test.cc
@@ -500,6 +500,14 @@ TEST_CASE(BigintDecStrings) {
         BigintOperations::ToDecimalCString(bigint, &ZoneAllocator);
     EXPECT_STREQ("291", str);
   }
+
+  {
+    const Bigint& bigint = Bigint::Handle(
+        BigintOperations::NewFromCString("100000000000000000000000000000000"));
+    const char* str =
+        BigintOperations::ToDecimalCString(bigint, &ZoneAllocator);
+    EXPECT_STREQ("100000000000000000000000000000000", str);
+  }
 }