Ignore zero length regions in the preview tool

The zero length regions are being produced when server indicates that
navigation ought to target an unnamed constructor, a closure, a library,
or any other unnamed element. This might be a bug in server's navigation
support, but I don't want to have to fix that before the preview tool
can be used in a UX study.

Change-Id: I35c9978d38937006ee00d76516f3ee607fca6850
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/124860
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

Author: bwilkerson

pkg/analysis_server/lib/src/edit/nnbd_migration/instrumentation_renderer.dart

@@ -265,37 +265,45 @@ class InstrumentationRenderer {
     // Compute insertions for navigation targets.
     //
     for (NavigationTarget region in unitInfo.targets) {
-      int openOffset = mapper.map(region.offset);
-      String openInsertion = openInsertions[openOffset] ?? '';
-      openInsertion = '<a id="o${region.offset}">$openInsertion';
-      openInsertions[openOffset] = openInsertion;
-
-      int closeOffset = openOffset + region.length;
-      String closeInsertion = closeInsertions[closeOffset] ?? '';
-      closeInsertion = '$closeInsertion</a>';
-      closeInsertions[closeOffset] = closeInsertion;
-    }
-    //
-    // Compute insertions for navigation sources, but skip the sources that
-    // point at themselves.
-    //
-    for (NavigationSource region in unitInfo.sources ?? <NavigationSource>[]) {
-      int openOffset = mapper.map(region.offset);
-      NavigationTarget target = region.target;
-      if (target.filePath != unitInfo.path || region.offset != target.offset) {
+      int regionLength = region.length;
+      if (regionLength > 0) {
+        int openOffset = mapper.map(region.offset);
         String openInsertion = openInsertions[openOffset] ?? '';
-        String htmlPath = pathContext.relative(pathMapper.map(target.filePath),
-            from: unitDir);
-        openInsertion = '<a href="$htmlPath#o${target.offset}">$openInsertion';
+        openInsertion = '<a id="o${region.offset}">$openInsertion';
         openInsertions[openOffset] = openInsertion;
 
-        int closeOffset = openOffset + region.length;
+        int closeOffset = openOffset + regionLength;
         String closeInsertion = closeInsertions[closeOffset] ?? '';
         closeInsertion = '$closeInsertion</a>';
         closeInsertions[closeOffset] = closeInsertion;
       }
     }
     //
+    // Compute insertions for navigation sources, but skip the sources that
+    // point at themselves.
+    //
+    for (NavigationSource region in unitInfo.sources ?? <NavigationSource>[]) {
+      int regionLength = region.length;
+      if (regionLength > 0) {
+        int openOffset = mapper.map(region.offset);
+        NavigationTarget target = region.target;
+        if (target.filePath != unitInfo.path ||
+            region.offset != target.offset) {
+          String openInsertion = openInsertions[openOffset] ?? '';
+          String htmlPath = pathContext
+              .relative(pathMapper.map(target.filePath), from: unitDir);
+          openInsertion =
+              '<a href="$htmlPath#o${target.offset}">$openInsertion';
+          openInsertions[openOffset] = openInsertion;
+
+          int closeOffset = openOffset + regionLength;
+          String closeInsertion = closeInsertions[closeOffset] ?? '';
+          closeInsertion = '$closeInsertion</a>';
+          closeInsertions[closeOffset] = closeInsertion;
+        }
+      }
+    }
+    //
     // Apply the insertions that have been computed.
     //
     List<int> offsets = []