Remove redundant variable descriptors from optimized code.

runtime/vm/debugger.cc

Index: runtime/vm/debugger.cc
===================================================================
--- runtime/vm/debugger.cc	(revision 45130)
+++ runtime/vm/debugger.cc	(working copy)
@@ -460,7 +460,12 @@
 
 void ActivationFrame::GetVarDescriptors() {
   if (var_descriptors_.IsNull()) {
-    var_descriptors_ = code().var_descriptors();
+    if (code().is_optimized()) {
+      Compiler::EnsureUnoptimizedCode(Thread::Current(), function());
+    }
+    var_descriptors_ = code().is_optimized()
+        ? Code::Handle(function().unoptimized_code()).var_descriptors()
+        : code().var_descriptors();

[srdjan, 2015/04/15 16:25:45]

Could this be just :
var_descriptors = Code::Handle(function().unoptimized_code()).var_descriptors(),
no need to check if code is optimized. 

We could store var_descriptors into function instead of unoptimized-code, but
that would mean larger footprint (var_descriptors alive as long as function is).

[Florian Schneider, 2015/04/16 11:37:18]

Yes. I'll simplify it.

I think it is fine to have it on unoptimized code. And since they are only
needed for variable inspection, normal stack traces should not be affected
performance-wise.

     ASSERT(!var_descriptors_.IsNull());
   }
 }
@@ -654,7 +659,7 @@
   return desc_indices_.length();
 }
 
-// TODO(hausner): Handle captured variables.
+
 RawObject* ActivationFrame::GetLocalVar(intptr_t slot_index) {
   if (deopt_frame_.IsNull()) {
     uword var_address = fp() + slot_index * kWordSize;