Fix issue 12718 - crash when the trying to access context variables in a frame.

runtime/vm/debugger.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/debugger.h"
 
 #include "include/dart_api.h"
 
 #include "vm/code_generator.h"
 #include "vm/code_patcher.h"
@@ skipping 517 matching lines @@
     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;
     intptr_t ctx_slot = var_info.index;
     if (level_diff == 0) {
-      *value = ctx_.At(ctx_slot);
+      // TODO(12767) : Need to ensure that we end up with the correct context
+      // here so that this check can be an assert.
+      if ((ctx_slot < ctx_.num_variables()) && (ctx_slot >= 0)) {
+        *value = ctx_.At(ctx_slot);
+      } else {
+        *value = Symbols::New("<unknown>");
+      }
     } else {
       ASSERT(level_diff > 0);
       Context& ctx = Context::Handle(ctx_.raw());
-      while (level_diff > 0) {
-        ASSERT(!ctx.IsNull());
+      while (level_diff > 0 && !ctx.IsNull()) {
         level_diff--;
         ctx = ctx.parent();
       }
-      ASSERT(!ctx.IsNull());
-      *value = ctx.At(ctx_slot);
+      // TODO(12767) : Need to ensure that we end up with the correct context
+      // here so that this check can be assert.
+      if (!ctx.IsNull() &&
+          ((ctx_slot < ctx_.num_variables()) && (ctx_slot >= 0))) {
+        *value = ctx.At(ctx_slot);
+      } else {
+        *value = Symbols::New("<unknown>");
+      }
     }
   }
 }
 
 
 RawArray* ActivationFrame::GetLocalVariables() {
   GetDescIndices();
   intptr_t num_variables = desc_indices_.length();
   String& var_name = String::Handle();
   Instance& value = Instance::Handle();
@@ skipping 1311 matching lines @@
 }
 
 
 void Debugger::RegisterCodeBreakpoint(CodeBreakpoint* bpt) {
   ASSERT(bpt->next() == NULL);
   bpt->set_next(code_breakpoints_);
   code_breakpoints_ = bpt;
 }
 
 }  // namespace dart