Use kFunctionCid instead of kIllegalCid to indicate that a generic fu…

…nction

type parameter does not parameterize a class, but a function.
Snapshot generation apparently trips on kIllegalCid.

R=johnmccutchan@google.com, rmacnak@google.com

Review-Url: https://codereview.chromium.org/2567163002 .

runtime/vm/object.cc

@@ -17610,7 +17610,7 @@ bool TypeParameter::IsEquivalent(const Instance& other, TrailPtr trail) const {
 
 void TypeParameter::set_parameterized_class(const Class& value) const {
   // Set value may be null.
-  classid_t cid = kIllegalCid;
+  classid_t cid = kFunctionCid;  // Denotes a function type parameter.
   if (!value.IsNull()) {
     cid = value.id();
   }
@@ -17625,7 +17625,7 @@ classid_t TypeParameter::parameterized_class_id() const {
 
 RawClass* TypeParameter::parameterized_class() const {
   classid_t cid = parameterized_class_id();
-  if (cid == kIllegalCid) {
+  if (cid == kFunctionCid) {
     return Class::null();
   }
   return Isolate::Current()->class_table()->At(cid);

runtime/vm/object.h

@@ -6057,7 +6057,7 @@ class TypeParameter : public AbstractType {
     return raw_ptr()->parameterized_function_;
   }
   bool IsClassTypeParameter() const {
-    return parameterized_class_id() != kIllegalCid;
+    return parameterized_class_id() != kFunctionCid;
   }
   bool IsFunctionTypeParameter() const {
     return parameterized_function() != Function::null();