Support unmapped areas in source maps.

pkg/source_maps/lib/builder.dart

Index: pkg/source_maps/lib/builder.dart
diff --git a/pkg/source_maps/lib/builder.dart b/pkg/source_maps/lib/builder.dart
index 12587cd84bf1ee2d235987e1310e36760b0016dc..ef22e3123720dcdf9d726c85662ac3b06c24cd61 100644
--- a/pkg/source_maps/lib/builder.dart
+++ b/pkg/source_maps/lib/builder.dart
@@ -75,15 +75,10 @@ class SourceMapBuilder {
       column = _append(buff, column, entry.target.column);
 
       // Encoding can be just the column offset if there is no source
-      // information, or if two consecutive mappings share exactly the same
-      // source information.
+      // information.
       var source = entry.source;
       if (source == null) continue;
       var newUrlId = _indexOf(_urls, source.sourceUrl);
-      if (newUrlId == srcUrlId && source.line == srcLine
-          && source.column == srcColumn && entry.identifierName == null) {
-        continue;
-      }
 
       srcUrlId = _append(buff, srcUrlId, newUrlId);
       srcLine = _append(buff, srcLine, source.line);