Add file identifiers to summaries.

pkg/analyzer/lib/src/summary/base.dart

Index: pkg/analyzer/lib/src/summary/base.dart
diff --git a/pkg/analyzer/lib/src/summary/base.dart b/pkg/analyzer/lib/src/summary/base.dart
index 1a1626ee9d65cee483e19f7c77efa4678d7e65c6..acdb8392aacaa34053523613fd7ad8de4a866b57 100644
--- a/pkg/analyzer/lib/src/summary/base.dart
+++ b/pkg/analyzer/lib/src/summary/base.dart
@@ -23,6 +23,20 @@ class Id {
 }
 
 /**
+ * Annotation used in the summary IDL to indicate that a summary class can be
+ * the top level object in an encoded summary.
+ */
+class TopLevel {
+  /**
+   * If non-null, identifier that will be stored in bytes 4-7 of the file,
+   * prior all other file data.  Must be exactly 4 Latin1 characters.
+   */
+  final String fileIdentifier;
+
+  const TopLevel([this.fileIdentifier]);
+}
+
+/**
  * Instances of this class represent data that has been read from a summary.
  */
 abstract class SummaryClass {