Fix for parsing default parameters.

pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart

 // Copyright (c) 2016, 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.
 
 library fasta.analyzer.ast_builder;
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/dart/ast/ast_factory.dart' show AstFactory;
 import 'package:analyzer/dart/ast/standard_ast_factory.dart' as standard;
 import 'package:analyzer/dart/ast/token.dart' as analyzer show Token;
@@ skipping 619 matching lines @@
         node = ast.fieldFormalParameter2(
             covariantKeyword: toAnalyzerToken(covariantKeyword),
             keyword: toAnalyzerToken(keyword),
             type: type,
             thisKeyword: toAnalyzerToken(thisKeyword),
             period: toAnalyzerToken(period),
             identifier: name);
       }
     }
 
-    if (defaultValue != null) {
-      node = ast.defaultFormalParameter(node, _toAnalyzerParameterKind(kind),
-          toAnalyzerToken(defaultValue.separator), defaultValue.value);
+    ParameterKind analyzerKind = _toAnalyzerParameterKind(kind);
+    if (analyzerKind != ParameterKind.REQUIRED) {
+      node = ast.defaultFormalParameter(node, analyzerKind,
+          toAnalyzerToken(defaultValue?.separator), defaultValue?.value);
     }
 
-    scope[name.name] = name.staticElement = new AnalyzerParameterElement(node);
+    if (name != null) {
+      scope[name.name] =
+          name.staticElement = new AnalyzerParameterElement(node);
+    }
     push(node);
   }
 
   @override
   void endFunctionTypedFormalParameter(
       Token covariantKeyword, Token thisKeyword, FormalParameterType kind) {
     debugEvent("FunctionTypedFormalParameter");
 
     FormalParameterList formalParameters = pop();
     TypeParameterList typeParameters = pop();
@@ skipping 827 matching lines @@
       } else if (identical('static', s)) {
         staticKeyword = token;
       } else if (identical('var', s)) {
         finalConstOrVarKeyword = token;
       } else {
         internalError('Unhandled modifier: $s');
       }
     }
   }
 }