Move a number of packages and some of pub over to using source_span.

sdk/lib/_internal/pub/lib/src/barback/transformer_isolate.dart

Index: sdk/lib/_internal/pub/lib/src/barback/transformer_isolate.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/transformer_isolate.dart b/sdk/lib/_internal/pub/lib/src/barback/transformer_isolate.dart
index fde485507a90067889efbaf185297727e91ed2cf..9a769e407a55ab875c03eb25153a0d57f2fcaf10 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/transformer_isolate.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/transformer_isolate.dart
@@ -9,7 +9,7 @@ import 'dart:convert';
 import 'dart:isolate';
 
 import 'package:barback/barback.dart';
-import 'package:source_maps/source_maps.dart';
+import 'package:source_span/source_span.dart';
 import 'package:stack_trace/stack_trace.dart';
 
 import '../../../asset/dart/serialize.dart';
@@ -122,11 +122,12 @@ class TransformerIsolate {
 }
 
 /// An error thrown when a transformer fails to load.
-class TransformerLoadError extends SpanException implements WrappedException {
+class TransformerLoadError extends SourceSpanException
+    implements WrappedException {
   final CrossIsolateException innerError;
   Chain get innerChain => innerError.stackTrace;
 
-  TransformerLoadError(CrossIsolateException error, Span span)
+  TransformerLoadError(CrossIsolateException error, SourceSpan span)
       : innerError = error,
         super("Error loading transformer: ${error.message}", span);
 }