VariableDescriptors contain more variables with Bytecode than in AST

[mkustermann]

For this example

class A {
  foo(unusedArg0) {
    baz() {
      for (int i = 0; i < 3; ++i) {
        print('[$i] $unusedArg0');
      }
    }
    baz();
  }
}

main() {
  A().foo(null);
}

With a small patch to the VM to dump variables:

diff --git a/runtime/vm/runtime_entry.cc b/runtime/vm/runtime_entry.cc
index 1bca9a114e..391ceee4a8 100644
--- a/runtime/vm/runtime_entry.cc
+++ b/runtime/vm/runtime_entry.cc
@@ -2315,9 +2315,20 @@ static void HandleStackOverflowTestCases(Thread* thread) {
         num_vars = frame->NumLocalVariables();
       }
 #endif
+      const bool print_vars = strstr(frame->function().ToFullyQualifiedCString(), "baz") != nullptr;
+      if (print_vars) {
+        THR_Print("STACK \n");
+      }
       TokenPosition unused = TokenPosition::kNoSource;
+      if (print_vars) {
+        THR_Print("    [%" Pd "] %s (num_vars: %d)\n",
+            i, frame->function().ToFullyQualifiedCString(), num_vars);
+      }
       for (intptr_t v = 0; v < num_vars; v++) {
         frame->VariableAt(v, &var_name, &unused, &unused, &unused, &var_value);
+        if (print_vars) {
+          THR_Print("       v[%" Pd "] %s\n", v, var_name.ToCString());
+        }
       }
     }
     if (FLAG_stress_async_stacks) {

We see a difference:

AST

% tools/build.py -mrelease -ax64 runtime_kernel
% out/ReleaseX64/dart --stacktrace_every=1 --deterministic test.dart
...
STACK 
    [3] file:///.../test.dart_A_foo_baz (num_vars: 2)
       v[0] unusedArg0
       v[1] i
STACK 
    [4] file:///.../test.dart_A_foo_baz (num_vars: 2)
       v[0] unusedArg0
       v[1] i
...

Bytecode:

% tools/gn.py -mrelease -ax64 --bytecode
% tools/build.py -mrelease -ax64 runtime_kernel
% out/ReleaseX64/dart --stacktrace_every=1 --deterministic  test.dart
STACK 
    [4] file:///.../test.dart_A_foo_baz (num_vars: 3)
       v[0] unusedArg0
       v[1] this
       v[2] i
STACK 
    [4] file:///.../test.dart_A_foo_baz (num_vars: 3)
       v[0] unusedArg0
       v[1] this
       v[2] i

It's unclear here why this shows up in the local variables for A_foo_baz.

/cc @alexmarkov @crelier