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

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

Index: sdk/lib/_internal/pub/lib/src/barback/transformer_id.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/transformer_id.dart b/sdk/lib/_internal/pub/lib/src/barback/transformer_id.dart
index e793ef3247b316e2c825189bb8b6b20433c8d4da..5d111ab034f1cea0f6e8ceb030bc6efb6bd93a6b 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/transformer_id.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/transformer_id.dart
@@ -8,7 +8,7 @@ import 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:path/path.dart' as p;
-import 'package:source_maps/source_maps.dart';
+import 'package:source_span/source_span.dart';
 
 import '../io.dart';
 import '../utils.dart';
@@ -35,7 +35,7 @@ class TransformerId {
   final String path;
 
   /// The source span from which this id was parsed.
-  final Span span;
+  final SourceSpan span;
 
   /// Whether this ID points to a built-in transformer exposed by pub.
   bool get isBuiltInTransformer => package.startsWith('\$');
@@ -47,7 +47,7 @@ class TransformerId {
   /// it just has a package name, it expands to lib/transformer.dart if that
   /// exists, or lib/${package}.dart otherwise. Otherwise, it expands to
   /// lib/${path}.dart. In either case it's located in the given package.
-  factory TransformerId.parse(String identifier, Span span) {
+  factory TransformerId.parse(String identifier, SourceSpan span) {
     if (identifier.isEmpty) {
       throw new FormatException('Invalid library identifier: "".');
     }
@@ -63,8 +63,8 @@ class TransformerId {
   TransformerId(this.package, this.path, this.span) {
     if (!package.startsWith('\$')) return;
     if (_BUILT_IN_TRANSFORMERS.contains(package)) return;
-    throw new SpanFormatException('Unsupported built-in transformer $package.',
-        span);
+    throw new SourceSpanFormatException(
+        'Unsupported built-in transformer $package.', span);
   }
 
   bool operator==(other) =>