Fixes bug where we would occasionally materialize a corrupted object.

runtime/vm/exceptions.cc

 // Copyright (c) 2011, 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/exceptions.h"
 
 #include "vm/dart_api_impl.h"
 #include "vm/dart_entry.h"
 #include "vm/debugger.h"
 #include "vm/flags.h"
@@ skipping 267 matching lines @@
 
 #if defined(USING_SIMULATOR)
   // Unwinding of the C++ frames and destroying of their stack resources is done
   // by the simulator, because the target stack_pointer is a simulated stack
   // pointer and not the C++ stack pointer.
 
   // Continue simulating at the given pc in the given frame after setting up the
   // exception object in the kExceptionObjectReg register and the stacktrace
   // object (may be raw null) in the kStackTraceObjectReg register.
   isolate->set_vm_tag(VMTag::kScriptTagId);
+  isolate->set_top_context(Context::null());
   Simulator::Current()->Longjmp(program_counter, stack_pointer, frame_pointer,
                                 raw_exception, raw_stacktrace);
 #else
   // Prepare for unwinding frames by destroying all the stack resources
   // in the previous frames.
 
   while (isolate->top_resource() != NULL &&
          (reinterpret_cast<uword>(isolate->top_resource()) < stack_pointer)) {
     isolate->top_resource()->~StackResource();
   }
 
   // Call a stub to set up the exception object in kExceptionObjectReg,
   // to set up the stacktrace object in kStackTraceObjectReg, and to
   // continue execution at the given pc in the given frame.
   typedef void (*ExcpHandler)(uword, uword, uword, RawObject*, RawObject*);
   ExcpHandler func = reinterpret_cast<ExcpHandler>(
       StubCode::JumpToExceptionHandlerEntryPoint());
 
   // Unpoison the stack before we tear it down in the generated stub code.
   uword current_sp = reinterpret_cast<uword>(&program_counter) - 1024;
   __asan_unpoison_memory_region(reinterpret_cast<void*>(current_sp),
                                 stack_pointer - current_sp);
   isolate->set_vm_tag(VMTag::kScriptTagId);
+  isolate->set_top_context(Context::null());
   func(program_counter, stack_pointer, frame_pointer,
        raw_exception, raw_stacktrace);
 #endif
   UNREACHABLE();
 }
 
 
 static RawField* LookupStacktraceField(const Instance& instance) {
   if (instance.GetClassId() < kNumPredefinedCids) {
     // 'class Error' is not a predefined class.
@@ skipping 397 matching lines @@
       break;
   }
 
   return DartLibraryCalls::InstanceCreate(library,
                                           *class_name,
                                           *constructor_name,
                                           arguments);
 }
 
 }  // namespace dart