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..1120b5f61c7101959b8249346223cf56728b6ad0 100644
--- a/runtime/vm/code_generator.cc
+++ b/runtime/vm/code_generator.cc
@@ -702,6 +702,7 @@ DEFINE_RUNTIME_ENTRY(BreakpointRuntimeHandler, 0) {
   DartFrameIterator iterator;
   StackFrame* caller_frame = iterator.NextFrame();
   ASSERT(caller_frame != NULL);
+  ASSERT(isolate->debugger() != NULL);
   uword orig_stub =
       isolate->debugger()->GetPatchedStubAddress(caller_frame->pc());
   isolate->debugger()->SignalBpReached();
@@ -1049,6 +1050,7 @@ DEFINE_RUNTIME_ENTRY(InvokeClosureNoSuchMethod, 3) {
 
 
 static bool CanOptimizeFunction(const Function& function, Isolate* isolate) {
+  ASSERT(isolate->debugger() != NULL);
   const intptr_t kLowInvocationCount = -100000000;
   if (isolate->debugger()->IsStepping() ||
       isolate->debugger()->HasBreakpoint(function)) {
@@ -1176,6 +1178,7 @@ DEFINE_RUNTIME_ENTRY(StackOverflow, 0) {
     DeoptimizeFunctionsOnStack();
   }
   if (do_stacktrace) {
+    ASSERT(isolate->debugger() != NULL);
     String& var_name = String::Handle();
     Instance& var_value = Instance::Handle();
     DebuggerStackTrace* stack = isolate->debugger()->StackTrace();