Do not enter the debugger when debugger is not active.

test/cctest/test-debug.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 7374 matching lines @@
   const char* script_1 = "debugger; throw new Error();";
   RunScriptInANewCFrame(script_1);
 
   // The second script uses forEach.
   const char* script_2 = "[0].forEach(function() { });";
   CompileRun(script_2);
 
   v8::Debug::SetDebugEventListener(NULL);
 }
 
+
+// Import from test-heap.cc
+int CountGlobalContexts();
+
+
+static void NopListener(v8::DebugEvent event,
+                        v8::Handle<v8::Object> exec_state,
+                        v8::Handle<v8::Object> event_data,
+                        v8::Handle<v8::Value> data) {
+}
+
+
+TEST(DebuggerCreatesContextIffActive) {
+  v8::HandleScope scope;
+  DebugLocalContext env;
+  CHECK_EQ(1, CountGlobalContexts());
+
+  v8::Debug::SetDebugEventListener(NULL);
+  CompileRun("debugger;");
+  CHECK_EQ(1, CountGlobalContexts());
+
+  v8::Debug::SetDebugEventListener(NopListener);
+  CompileRun("debugger;");
+  CHECK_EQ(2, CountGlobalContexts());
+
+  v8::Debug::SetDebugEventListener(NULL);
+}
+
 #endif  // ENABLE_DEBUGGER_SUPPORT