run.sh

d8 --allow-natives-syntax test.js

test.js

function foo() {
    var a = true;
    var b = { "prop": "you" };
    var c = "Bad Ass";
    var d = ["my", "life", 0];

    var locals = %LocalVariabels(foo);
    print(JSON.stringify(locals));
}

foo();

v8.diff

index 08bc92c..393cc9a 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -14552,6 +14552,61 @@ RUNTIME_FUNCTION(Runtime_GetScript) {
 }


+// Get variabels in this function scope
+RUNTIME_FUNCTION(Runtime_LocalVariabels) {
+  HandleScope scope(isolate);
+
+  DCHECK(args.length() == 1);
+
+  CONVERT_ARG_HANDLE_CHECKED(JSFunction, function, 0);
+
+  // Unpack the function scope
+  v8::Isolate* v8_isolate = reinterpret_cast<v8::Isolate*>(isolate);
+  SharedFunctionInfo* shared = function->shared();
+  ScopeInfo* scope_info = shared->scope_info();
+  Object* script_obj = shared->script();
+
+  if (script_obj->IsScript()) {
+    // Search the frames for a function match
+    JavaScriptFrame* frame;
+    bool found = false;
+
+    for (JavaScriptFrameIterator it(isolate); !it.done(); it.Advance()) {
+      frame = it.frame();
+      if (frame->function() == *function.location()) {
+        found = true;
+        break;
+      }
+    }
+
+    // function is not in active scope, stop here
+    if (!found) {
+      return isolate->heap()->undefined_value();
+    }
+
+    // Build an object containing the local variables
+    Local<v8::Object> locals = v8::Object::New(v8_isolate);
+
+    int stack_local_count = scope_info->StackLocalCount();
+
+    for (int i = 0; i < stack_local_count; i++) {
+      String* name = scope_info->StackLocalName(i);
+      const char* name_buf = name->ToCString().get();
+
+      locals->Set(
+        v8::Handle<v8::Value>::Cast(reinterpret_cast<v8::Handle<v8::String>>(handle(reinterpret_cast<v8::String*>(name), isolate))),
+        v8::Handle<v8::Value>::Cast(reinterpret_cast<v8::Handle<v8::Object>>(handle(reinterpret_cast<v8::Object*>(frame->GetExpression(i)), isolate)))
+      );
+      printf("print :: name: %s\n", name_buf);
+    }
+
+    return reinterpret_cast<Object*>(*locals);
+  }
+
+  return isolate->heap()->undefined_value();
+}

The casting on L50-51 is completely messed up, and won't compile.
But basically we need to go
from: v8::internal::String* to v8::Handle<v8::Value>
from: v8::internal::Object* to v8::Handle<v8::Value>
or find some other way of creating an object and setting the properties.

I have confirmed that scope_info->StackLocalName(i); returns the variable name, I believe that frame->GetExpression(i) returns the variable value.

@AndreasMadsen Wow, that's a lot of progress! I've not been able to get anywhere near that