Disable stack context when its frame is for optimized code.

runtime/vm/debugger.cc

Index: runtime/vm/debugger.cc
===================================================================
--- runtime/vm/debugger.cc	(revision 19823)
+++ runtime/vm/debugger.cc	(working copy)
@@ -304,7 +304,13 @@
 
 // Calculate the context level at the current token index of the frame.
 intptr_t ActivationFrame::ContextLevel() {
-  if (context_level_ < 0) {
+  if (context_level_ < 0 && !ctx_.IsNull()) {
+    if (code_.is_optimized()) {
+      // Set context to null, since this and all subsequent frames are
+      // invalid.
+      SetContext(Context::Handle());
+      return context_level_;
+    }
     context_level_ = 0;
     intptr_t pc_desc_idx = PcDescIndex();
     ASSERT(!pc_desc_.IsNull());
@@ -493,13 +499,13 @@
   if (var_info.kind == RawLocalVarDescriptors::kStackVar) {
     *value = GetLocalVarValue(var_info.index);
   } else {
-    // TODO(tball): enable context variables once problem with VariableAt() is
-    // fixed, where frame_ctx_level is sometimes off by 1 (issues 8593 and 8594)
-    /*
     ASSERT(var_info.kind == RawLocalVarDescriptors::kContextVar);
-    ASSERT(!ctx_.IsNull());
     // The context level at the PC/token index of this activation frame.
     intptr_t frame_ctx_level = ContextLevel();
+    if (ctx_.IsNull()) {
+      *value = Symbols::New("<unknown>");
+      return;
+    }
     // The context level of the variable.
     intptr_t var_ctx_level = var_info.scope_id;
     intptr_t level_diff = frame_ctx_level - var_ctx_level;
@@ -516,8 +522,7 @@
       }
       ASSERT(!ctx.IsNull());
       *value = ctx.At(ctx_slot);
-    } */
-    *value = Symbols::New("<unknown>");
+    }
   }
 }