[bigint] Make ToBigInt throw the correct error.

We must throw a SyntaxError only when failing to convert a string. In
the other cases we must throw a TypeError.

R=jkummerow@chromium.org

Bug: v8:6791
Change-Id: I802d8b6830b341f87e46e7de198af74ba95b8658
Reviewed-on: https://chromium-review.googlesource.com/752803Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49131}

src/objects/bigint.cc

@@ -530,14 +530,16 @@ MaybeHandle<BigInt> BigInt::FromObject(Isolate* isolate, Handle<Object> obj) {
   }
   if (obj->IsString()) {
     Handle<BigInt> n;
-    if (StringToBigInt(isolate, Handle<String>::cast(obj)).ToHandle(&n)) {
-      return n;
+    if (!StringToBigInt(isolate, Handle<String>::cast(obj)).ToHandle(&n)) {
+      THROW_NEW_ERROR(isolate,
+                      NewSyntaxError(MessageTemplate::kBigIntFromObject, obj),
+                      BigInt);
     }
-    // ... else fall through.
+    return n;
   }
 
   THROW_NEW_ERROR(
-      isolate, NewSyntaxError(MessageTemplate::kBigIntFromObject, obj), BigInt);
+      isolate, NewTypeError(MessageTemplate::kBigIntFromObject, obj), BigInt);
 }
 
 Handle<Object> BigInt::ToNumber(Handle<BigInt> x) {

test/mjsunit/harmony/bigint/basics.js

@@ -22,8 +22,8 @@ const six = BigInt(6);
 
 // ToBigInt, NumberToBigInt, BigInt
 {
-  assertThrows(() => BigInt(undefined), SyntaxError);
-  assertThrows(() => BigInt(null), SyntaxError);
+  assertThrows(() => BigInt(undefined), TypeError);
+  assertThrows(() => BigInt(null), TypeError);
   assertThrows(() => BigInt({}), SyntaxError);
   assertThrows(() => BigInt("foo"), SyntaxError);
 }{