Fix how spans are rendered in the debug output. (#1168)

A while back, I did a big refactoring where each Chunk's split now
describes the line split before that Chunk's text instead of after.
When I did that, it looks like I didn't update how the debug output
draws spans.

Since this is only used for debugging the formatter, it doesn't affect
users. But it make for some very confusing debugging for me.

Author: munificent

lib/src/debug.dart

@@ -127,26 +127,31 @@ void dumpChunks(int start, List<Chunk> chunks) {
       var spanBars = '';
       for (var span in spans) {
         if (chunk.spans.contains(span)) {
-          if (index == 0 || !chunks[index - 1].spans.contains(span)) {
+          if (index == chunks.length - 1 ||
+              !chunks[index + 1].spans.contains(span)) {
+            // This is the last chunk with the span.
+            spanBars += '╙';
+          } else {
+            spanBars += '║';
+          }
+        } else {
+          // If the next chunk has this span, then show it bridging this chunk
+          // and the next because a split between them breaks the span.
+          if (index < chunks.length - 1 &&
+              chunks[index + 1].spans.contains(span)) {
             if (span.cost == 1) {
               spanBars += '╓';
             } else {
               spanBars += span.cost.toString();
             }
-          } else {
-            spanBars += '║';
-          }
-        } else {
-          if (index > 0 && chunks[index - 1].spans.contains(span)) {
-            spanBars += '╙';
-          } else {
-            spanBars += ' ';
           }
         }
       }
       row.add(spanBars);
     }
 
+    row.add(chunk.spans.map((span) => span.id).join(' '));
+
     if (chunk.text.length > 70) {
       row.add(chunk.text.substring(0, 70));
     } else {