[bigint] Fix bug in exponentiation.

R=jkummerow@chromium.org

Bug: v8:7505, v8:6791
Change-Id: I11b0031dfafa499a813e3e52080ee5542224799a
Reviewed-on: https://chromium-review.googlesource.com/941130Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51639}

src/objects/bigint.cc

@@ -321,11 +321,10 @@ MaybeHandle<BigInt> BigInt::Exponentiate(Handle<BigInt> base,
   // 3. Return a BigInt representing the mathematical value of base raised
   //    to the power exponent.
   if (base->is_zero()) return base;
-  if (base->length() == 1 && base->digit(0) == 1) return base;
   // For all bases >= 2, very large exponents would lead to unrepresentable
   // results.
   STATIC_ASSERT(kMaxLengthBits < std::numeric_limits<digit_t>::max());
-  if (exponent->length() > 1) {
+  if (!(base->length() == 1 && base->digit(0) == 1) && exponent->length() > 1) {
     THROW_NEW_ERROR(isolate, NewRangeError(MessageTemplate::kBigIntTooBig),
                     BigInt);
   }

test/mjsunit/harmony/bigint/exp.js

@@ -4,6 +4,12 @@
 
 // Flags: --allow-natives-syntax --harmony-bigint
 
+assertEquals(1n, (-1n) ** 0n);
+assertEquals(-1n, (-1n) ** 1n);
+assertEquals(1n, (-1n) ** 2n);
+assertEquals(-1n, (-1n) ** 3n);
+assertEquals(1n, (-1n) ** 4n);
+
 assertEquals(1n, 0n ** 0n);
 assertEquals(0n, 0n ** 1n);
 assertEquals(0n, 0n ** 23n);