Add file identifiers to summaries.

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

Index: pkg/analyzer/lib/src/summary/format.dart
diff --git a/pkg/analyzer/lib/src/summary/format.dart b/pkg/analyzer/lib/src/summary/format.dart
index 99dbbcea2a24038fb950f6ea87e018e45c9131a3..2a7ada2f3a8bacb18986b042d06661486cfc236d 100644
--- a/pkg/analyzer/lib/src/summary/format.dart
+++ b/pkg/analyzer/lib/src/summary/format.dart
@@ -656,7 +656,7 @@ class LinkedLibraryBuilder extends Object with _LinkedLibraryMixin implements id
 
   List<int> toBuffer() {
     fb.Builder fbBuilder = new fb.Builder();
-    return fbBuilder.finish(finish(fbBuilder));
+    return fbBuilder.finish(finish(fbBuilder), "LLib");
   }
 
   fb.Offset finish(fb.Builder fbBuilder) {
@@ -1213,7 +1213,7 @@ class PackageBundleBuilder extends Object with _PackageBundleMixin implements id
 
   List<int> toBuffer() {
     fb.Builder fbBuilder = new fb.Builder();
-    return fbBuilder.finish(finish(fbBuilder));
+    return fbBuilder.finish(finish(fbBuilder), "PBdl");
   }
 
   fb.Offset finish(fb.Builder fbBuilder) {
@@ -4969,7 +4969,7 @@ class UnlinkedPublicNamespaceBuilder extends Object with _UnlinkedPublicNamespac
 
   List<int> toBuffer() {
     fb.Builder fbBuilder = new fb.Builder();
-    return fbBuilder.finish(finish(fbBuilder));
+    return fbBuilder.finish(finish(fbBuilder), "UPNS");
   }
 
   fb.Offset finish(fb.Builder fbBuilder) {
@@ -5782,7 +5782,7 @@ class UnlinkedUnitBuilder extends Object with _UnlinkedUnitMixin implements idl.
 
   List<int> toBuffer() {
     fb.Builder fbBuilder = new fb.Builder();
-    return fbBuilder.finish(finish(fbBuilder));
+    return fbBuilder.finish(finish(fbBuilder), "UUnt");
   }
 
   fb.Offset finish(fb.Builder fbBuilder) {