Introduce code to generate summaries from an element model.

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 a997f980a73e87ed4a5280d6aca2a4106d2b8d64..56d555697592368f3318d7e9c2593c2f86d04750 100644
--- a/pkg/analyzer/lib/src/summary/format.dart
+++ b/pkg/analyzer/lib/src/summary/format.dart
@@ -7,6 +7,7 @@
 
 library analyzer.src.summary.format;
 
+import 'dart:convert';
 import 'builder.dart' as builder;
 
 enum PrelinkedReferenceKind {
@@ -71,6 +72,8 @@ class PrelinkedLibrary {
       _importDependencies = json["importDependencies"],
       _references = json["references"]?.map((x) => new PrelinkedReference.fromJson(x))?.toList();
 
+  PrelinkedLibrary.fromBuffer(List<int> buffer) : this.fromJson(JSON.decode(UTF8.decode(buffer)));
+
   UnlinkedLibrary get unlinked => _unlinked;
   List<PrelinkedDependency> get dependencies => _dependencies ?? const <PrelinkedDependency>[];
   List<int> get importDependencies => _importDependencies ?? const <int>[];