fix new strong mode error

pkgs/source_span/lib/src/span_mixin.dart

diff --git a/pkgs/source_span/lib/src/span_mixin.dart b/pkgs/source_span/lib/src/span_mixin.dart
index f6d04fa..a258cf5 100644
--- a/pkgs/source_span/lib/src/span_mixin.dart
+++ b/pkgs/source_span/lib/src/span_mixin.dart
@@ -60,7 +60,7 @@
     if (length == 0 && this is! SourceSpanWithContext) return buffer.toString();
     buffer.write("\n");
 
-    var textLine;
+    String textLine;
     if (this is SourceSpanWithContext) {
       var context = (this as SourceSpanWithContext).context;
       var lineStart = findLineStart(context, text, column);