Rework recent changes to LineInfo to be backward compatible.

pkg/analysis_server/lib/src/status/get_handler.dart

Index: pkg/analysis_server/lib/src/status/get_handler.dart
diff --git a/pkg/analysis_server/lib/src/status/get_handler.dart b/pkg/analysis_server/lib/src/status/get_handler.dart
index b97ef3ba678916c8a9823a5119f2e6f14f049372..b20d6426102bac017c8d832cb63fc0ecce846657 100644
--- a/pkg/analysis_server/lib/src/status/get_handler.dart
+++ b/pkg/analysis_server/lib/src/status/get_handler.dart
@@ -691,15 +691,16 @@ class GetHandler {
             int lineCount(Set<Source> sources, bool explicit) {
               return sources.fold(0, (int previousTotal, Source source) {
                 LineInfo lineInfo = context.getLineInfo(source);
-                if (lineInfo == null) {
-                  return previousTotal;
-                }
-                if (explicit) {
-                  explicitLineInfoCount++;
+                if (lineInfo is LineInfoWithCount) {
+                  if (explicit) {
+                    explicitLineInfoCount++;
+                  } else {
+                    implicitLineInfoCount++;
+                  }
+                  return previousTotal + lineInfo.lineCount;
                 } else {
-                  implicitLineInfoCount++;
+                  return previousTotal;
                 }
-                return previousTotal + lineInfo.lineCount;
               });
             }
             explicitSourceCount += explicitSources.length;