Roll Observatory packages and add a roll script

packages/source_span/lib/src/file.dart

Index: packages/source_span/lib/src/file.dart
diff --git a/source_span/lib/src/file.dart b/packages/source_span/lib/src/file.dart
similarity index 99%
rename from source_span/lib/src/file.dart
rename to packages/source_span/lib/src/file.dart
index 95fa92ca3b12a7586ca4d3e995c9407039cd69f1..37790ce9296b8d8a31104373a422e36e22841aa9 100644
--- a/source_span/lib/src/file.dart
+++ b/packages/source_span/lib/src/file.dart
@@ -246,6 +246,9 @@ abstract class FileSpan implements SourceSpanWithContext {
   /// The [file] that [this] belongs to.
   SourceFile get file;
 
+  FileLocation get start;
+  FileLocation get end;
+
   /// Returns a new span that covers both [this] and [other].
   ///
   /// Unlike [union], [other] may be disjoint from [this]. If it is, the text