Fixes crashes in VM isolate shutdown.

runtime/vm/service.cc

Index: runtime/vm/service.cc
diff --git a/runtime/vm/service.cc b/runtime/vm/service.cc
index 0593f7faa5edd29610cd8a5c1f164c3c2c2aecf0..6fb3470f4e734fbf37f1b96f0e17e4f359e7a98b 100644
--- a/runtime/vm/service.cc
+++ b/runtime/vm/service.cc
@@ -780,6 +780,7 @@ static const MethodParameter* get_stack_params[] = {
 
 
 static bool GetStack(Isolate* isolate, JSONStream* js) {
+  ASSERT(isolate->debugger() != NULL);
   DebuggerStackTrace* stack = isolate->debugger()->StackTrace();
   // Do we want the complete script object and complete local variable objects?
   // This is true for dump requests.
@@ -1255,6 +1256,7 @@ static void PrintSentinel(JSONStream* js, SentinelType sentinel_type) {
 
 
 static Breakpoint* LookupBreakpoint(Isolate* isolate, const char* id) {
+  ASSERT(isolate->debugger() != NULL);
   size_t end_pos = strcspn(id, "/");
   if (end_pos == strlen(id)) {
     return NULL;
@@ -1621,6 +1623,7 @@ static const MethodParameter* evaluate_in_frame_params[] = {
 
 
 static bool EvaluateInFrame(Isolate* isolate, JSONStream* js) {
+  ASSERT(isolate->debugger() != NULL);
   DebuggerStackTrace* stack = isolate->debugger()->StackTrace();
   intptr_t framePos = UIntParameter::Parse(js->LookupParam("frameIndex"));
   if (framePos > stack->Length()) {
@@ -1856,6 +1859,7 @@ static const MethodParameter* add_breakpoint_params[] = {
 
 
 static bool AddBreakpoint(Isolate* isolate, JSONStream* js) {
+  ASSERT(isolate->debugger() != NULL);
   const char* line_param = js->LookupParam("line");
   intptr_t line = UIntParameter::Parse(line_param);
   const char* script_id = js->LookupParam("scriptId");
@@ -1887,6 +1891,7 @@ static const MethodParameter* add_breakpoint_at_entry_params[] = {
 
 
 static bool AddBreakpointAtEntry(Isolate* isolate, JSONStream* js) {
+  ASSERT(isolate->debugger() != NULL);
   const char* function_id = js->LookupParam("functionId");
   Object& obj = Object::Handle(LookupHeapObject(isolate, function_id, NULL));
   if (obj.raw() == Object::sentinel().raw() || !obj.IsFunction()) {
@@ -1915,6 +1920,7 @@ static const MethodParameter* add_breakpoint_at_activation_params[] = {
 
 
 static bool AddBreakpointAtActivation(Isolate* isolate, JSONStream* js) {
+  ASSERT(isolate->debugger() != NULL);
   const char* object_id = js->LookupParam("objectId");
   Object& obj = Object::Handle(LookupHeapObject(isolate, object_id, NULL));
   if (obj.raw() == Object::sentinel().raw() || !obj.IsInstance()) {
@@ -1942,6 +1948,7 @@ static const MethodParameter* remove_breakpoint_params[] = {
 
 
 static bool RemoveBreakpoint(Isolate* isolate, JSONStream* js) {
+  ASSERT(isolate->debugger() != NULL);
   if (!js->HasParam("breakpointId")) {
     PrintMissingParamError(js, "breakpointId");
     return true;
@@ -2147,6 +2154,7 @@ static const MethodParameter* resume_params[] = {
 
 
 static bool Resume(Isolate* isolate, JSONStream* js) {
+  ASSERT(isolate->debugger() != NULL);
   const char* step_param = js->LookupParam("step");
   if (isolate->message_handler()->paused_on_start()) {
     isolate->message_handler()->set_pause_on_start(false);