[vm] Consider a reload with no changed files as a reload for _getUnusedChangesInLastReload.

Add missing messages for various vm-service error codes.

Bug: https://github.com/flutter/flutter/issues/12300
Change-Id: I1f3607f94c5ab2fb770813f0502c3b1c9c40fdf4
Reviewed-on: https://dart-review.googlesource.com/11642
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Siva Annamalai <asiva@google.com>

runtime/observatory/tests/service/unused_changes_in_last_reload_test.dart

@@ -54,9 +54,9 @@ var tests = [
       print(e);
     });
 
-    // Reload to v2.
+    // Reload to v1 (null change).
     var response = await childIsolate.reloadSources(
-      rootLibUri: v2Uri.toString(),
+      rootLibUri: v1Uri.toString(),
     );
     print(response);
     expect(response['success'], isTrue);
@@ -65,6 +65,19 @@ var tests = [
     response = await childIsolate.invokeRpc("_getUnusedChangesInLastReload", {});
     print(response);
     var unused = response['unused'].map((ea) => ea.toString());
+    expect(unused, unorderedEquals([]));
+
+    // Reload to v2.
+    response = await childIsolate.reloadSources(
+      rootLibUri: v2Uri.toString(),
+    );
+    print(response);
+    expect(response['success'], isTrue);
+
+    // Fetch unused.
+    response = await childIsolate.invokeRpc("_getUnusedChangesInLastReload", {});
+    print(response);
+    unused = response['unused'].map((ea) => ea.toString());
     expect(unused, unorderedEquals([
       'Class(C)',
       'Class(NewClass)',

runtime/vm/isolate_reload.cc

@@ -600,6 +600,9 @@ void IsolateReloadContext::Reload(bool force_reload,
   if (!modified_libs_->Contains(old_root_lib.index())) {
     ASSERT(modified_libs_->IsEmpty());
     reload_skipped_ = true;
+    // Inform GetUnusedChangesInLastReload that a reload has happened.
+    I->object_store()->set_changed_in_last_reload(
+        GrowableObjectArray::Handle(GrowableObjectArray::New()));
     ReportOnJSON(js_);
     TIR_Print("---- SKIPPING RELOAD (No libraries were modified)\n");
     return;

runtime/vm/json_stream.cc

@@ -138,6 +138,10 @@ static const char* GetJSONRpcErrorMessage(intptr_t code) {
       return "Feature is disabled";
     case kCannotAddBreakpoint:
       return "Cannot add breakpoint";
+    case kStreamAlreadySubscribed:
+      return "Stream already subscribed";
+    case kStreamNotSubscribed:
+      return "Stream not subscribed";
     case kIsolateMustBeRunnable:
       return "Isolate must be runnable";
     case kIsolateMustBePaused:
@@ -146,14 +150,16 @@ static const char* GetJSONRpcErrorMessage(intptr_t code) {
       return "Cannot resume execution";
     case kIsolateIsReloading:
       return "Isolate is reloading";
+    case kIsolateReloadBarred:
+      return "Isolate cannot be reloaded";
+    case kIsolateMustHaveReloaded:
+      return "Isolate must have reloaded";
     case kFileSystemAlreadyExists:
       return "File system already exists";
     case kFileSystemDoesNotExist:
       return "File system does not exist";
     case kFileDoesNotExist:
       return "File does not exist";
-    case kIsolateReloadBarred:
-      return "Isolate cannot be reloaded";
     default:
       return "Extension error";
   }