Fixes crashes in VM isolate shutdown.

runtime/vm/code_generator.cc

Index: runtime/vm/code_generator.cc
diff --git a/runtime/vm/code_generator.cc b/runtime/vm/code_generator.cc
index 4485c5805227d4a233ff785db0b9a708b42021e8..2e358d3c763cefe991e0a05889c773d6e17dc1d1 100644
--- a/runtime/vm/code_generator.cc
+++ b/runtime/vm/code_generator.cc
@@ -698,7 +698,6 @@ static void CheckResultError(const Object& result) {
 // Gets called from debug stub when code reaches a breakpoint
 // set on a runtime stub call.
 DEFINE_RUNTIME_ENTRY(BreakpointRuntimeHandler, 0) {
-  ASSERT(isolate->debugger() != NULL);
   DartFrameIterator iterator;
   StackFrame* caller_frame = iterator.NextFrame();
   ASSERT(caller_frame != NULL);
@@ -711,7 +710,6 @@ DEFINE_RUNTIME_ENTRY(BreakpointRuntimeHandler, 0) {
 
 
 DEFINE_RUNTIME_ENTRY(SingleStepHandler, 0) {
-  ASSERT(isolate->debugger() != NULL);
   isolate->debugger()->DebuggerStepCallback();
 }