Ensure that summary builders aren't used after call to finish()

pkg/analyzer/tool/summary/generate.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * This file contains code to generate serialization/deserialization logic for
  * summaries based on an "IDL" description of the summary format (written in
  * stylized Dart).
  *
  * For each class in the "IDL" input, two corresponding classes are generated:
@@ skipping 289 matching lines @@
           out('${dartType(type)} get $fieldName => _$fieldName$defaultSuffix;');
         });
       });
       out('}');
       out();
       List<String> builderParams = <String>[];
       out('class ${name}Builder {');
       indent(() {
         out('final Map _json = {};');
         out();
+        out('bool _finished = false;');
+        out();
         out('${name}Builder(builder.BuilderContext context);');
         cls.fields.forEach((String fieldName, idlModel.FieldType type) {
           out();
           String conversion = '_value';
           String condition = '';
           if (type.isList) {
             if (_idl.classes.containsKey(type.typeName)) {
               conversion = '$conversion.map((b) => b.finish()).toList()';
             } else {
               conversion = '$conversion.toList()';
             }
             condition = ' || _value.isEmpty';
           } else if (_idl.enums.containsKey(type.typeName)) {
             conversion = '$conversion.index';
             condition = ' || _value == ${defaultValue(type)}';
           } else if (_idl.classes.containsKey(type.typeName)) {
             conversion = '$conversion.finish()';
           }
           builderParams.add('${encodedType(type)} $fieldName');
           out('void set $fieldName(${encodedType(type)} _value) {');
           indent(() {
+            out('assert(!_finished);');
             out('assert(!_json.containsKey(${quoted(fieldName)}));');
             out('if (_value != null$condition) {');
             indent(() {
               out('_json[${quoted(fieldName)}] = $conversion;');
             });
             out('}');
           });
           out('}');
         });
         if (cls.isTopLevel) {
           out();
           out('List<int> toBuffer() => UTF8.encode(JSON.encode(finish()));');
         }
         out();
-        out('Map finish() => _json;');
+        out('Map finish() {');
+        indent(() {
+          out('assert(!_finished);');
+          out('_finished = true;');
+          out('return _json;');
+        });
+        out('}');
       });
       out('}');
       out();
       out('${name}Builder encode$name(builder.BuilderContext builderContext, {${builderParams.join(', ')}}) {');
       indent(() {
         out('${name}Builder builder = new ${name}Builder(builderContext);');
         cls.fields.forEach((String fieldName, idlModel.FieldType type) {
           out('builder.$fieldName = $fieldName;');
         });
         out('return builder;');
       });
       out('}');
       out();
     });
   }
 
   /**
    * Enclose [s] in quotes, escaping as necessary.
    */
   String quoted(String s) {
     return JSON.encode(s);
   }
 }