Display isolate message queue in Observatory debugger

runtime/vm/service_test.cc

Index: runtime/vm/service_test.cc
diff --git a/runtime/vm/service_test.cc b/runtime/vm/service_test.cc
index 31efafc915864179d618b3a9375d612a71d321cf..c59c4c69aef303fffd48e4911094d93a01f636a7 100644
--- a/runtime/vm/service_test.cc
+++ b/runtime/vm/service_test.cc
@@ -506,6 +506,7 @@ TEST_CASE(Service_EmbedderRootHandler) {
   EXPECT_STREQ("{\"result\":beta, \"id\":\"0\"}", handler.msg());
 }
 
+
 TEST_CASE(Service_EmbedderIsolateHandler) {
   const char* kScript =
     "var port;\n"  // Set to our mock port by C++.
@@ -544,6 +545,33 @@ TEST_CASE(Service_EmbedderIsolateHandler) {
 }
 
 
+static uint8_t* AllocMsg(const char* str) {
+  return reinterpret_cast<uint8_t*>(strdup(str));
+}
+
+
+TEST_CASE(Service_MessageQueue) {
+  MessageQueue queue;
+  EXPECT(queue.IsEmpty());
+
+  Dart_Port port = 1;
+
+  const char* str1 = "msg1";
+
+  // Add a message.
+  Message* msg1 = new Message(
+      port, AllocMsg(str1), strlen(str1) + 1, Message::kNormalPriority);
+  queue.Enqueue(msg1, false);
+  EXPECT(queue.Length() == 1);
+  EXPECT(!queue.IsEmpty());
+
+  // Print MessageQueue to JSON.
+  JSONStream stream;
+  queue.PrintJSON(&stream);
+  // Expect a Message type.
+  EXPECT_SUBSTRING("\"type\":\"Message\"", stream.ToCString());
+}
+
 // TODO(zra): Remove when tests are ready to enable.
 #if !defined(TARGET_ARCH_ARM64)