[vm] Support non-class 'constructors' when calculating const coverage

With https://dart-review.googlesource.com/c/sdk/+/461160 const coverage
"constructors" can include extension type "constructors".
The CL was reverted
(https://dart-review.googlesource.com/c/sdk/+/461480) because it made
the VM crash.
This CL should make it work.

TEST=covered by existing tests
Change-Id: I3a24932a60cc97780e1a1f27bef474e825172e0e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/461500
Commit-Queue: Jens Johansen <jensj@google.com>
Reviewed-by: Slava Egorov <vegorov@google.com>

Author: jensjoha

runtime/vm/kernel.cc

@@ -234,18 +234,19 @@ ArrayPtr Script::CollectConstConstructorCoverageFromKernel() const {
 
   kernel::KernelReaderHelper kernel_reader(zone, &helper, data, 0);
 
-  // Read "constant coverage constructors".
+  // Read "constant coverage constructors". Note that this also includes
+  // "constructors" for extensions type which is actually toplevel methods.
   const intptr_t constant_coverage_constructors =
       kernel_reader.ReadListLength();
   const Array& constructors =
       Array::Handle(Array::New(constant_coverage_constructors));
+  Object& member = Object::Handle(zone);
+  Function& target = Function::Handle(zone);
   for (intptr_t i = 0; i < constant_coverage_constructors; ++i) {
     kernel::NameIndex kernel_name = kernel_reader.ReadCanonicalNameReference();
-    Class& klass = Class::ZoneHandle(
-        zone,
-        helper.LookupClassByKernelClass(helper.EnclosingName(kernel_name)));
-    const Function& target = Function::ZoneHandle(
-        zone, helper.LookupConstructorByKernelConstructor(klass, kernel_name));
+    member = helper.LookupMemberByMember(kernel_name);
+    ASSERT(member.IsFunction());
+    target = Function::Cast(member).ptr();
     constructors.SetAt(i, target);
   }
   return constructors.ptr();