Fix behavior causing frames lacking a source map to always be omitted. Add `includeUnmappedFrames` …

lib/source_map_stack_trace.dart

Index: lib/source_map_stack_trace.dart
diff --git a/lib/source_map_stack_trace.dart b/lib/source_map_stack_trace.dart
index ae2f124d37219bde279cf5fb6d49e8ae8327627b..c9e49f20f68d6b680657157f38098c4afb78d908 100644
--- a/lib/source_map_stack_trace.dart
+++ b/lib/source_map_stack_trace.dart
@@ -62,7 +62,8 @@ StackTrace mapStackTrace(Mapping sourceMap, StackTrace stackTrace,
 
     // Subtract 1 because stack traces use 1-indexed lines and columns and
     // source maps uses 0-indexed.
-    var span = sourceMap.spanFor(frame.line - 1, column - 1);
+    var span = sourceMap.spanFor(frame.line - 1, column - 1,
+        uri: frame.uri?.toString());
 
     // If we can't find a source span, ignore the frame. It's probably something
     // internal that the user doesn't care about.