Add flag to track if a constructor is the synthetic default constructor.

pkg/kernel/lib/ast.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.
 
 /// -----------------------------------------------------------------------
 ///                           ERROR HANDLING
 /// -----------------------------------------------------------------------
 ///
 /// As a rule of thumb, errors that can be detected statically are handled by
 /// the frontend, typically by translating the erroneous code into a 'throw' or
@@ skipping 920 matching lines @@
 /// For unnamed constructors, the name is an empty string (in a [Name]).
 class Constructor extends Member {
   int flags = 0;
   FunctionNode function;
   List<Initializer> initializers;
 
   Constructor(this.function,
       {Name name,
       bool isConst: false,
       bool isExternal: false,
+      bool isSyntheticDefault: false,
       List<Initializer> initializers,
       int transformerFlags: 0,
       Reference reference})
       : this.initializers = initializers ?? <Initializer>[],
         super(name, reference) {
     function?.parent = this;
     setParents(this.initializers, this);
     this.isConst = isConst;
     this.isExternal = isExternal;
+    this.isSyntheticDefault = isSyntheticDefault;
     this.transformerFlags = transformerFlags;
   }
 
   static const int FlagConst = 1 << 0; // Must match serialized bit positions.
   static const int FlagExternal = 1 << 1;
+  static const int FlagSyntheticDefault = 1 << 2;
 
   bool get isConst => flags & FlagConst != 0;
   bool get isExternal => flags & FlagExternal != 0;
 
+  /// True if this is a synthetic default constructor inserted in a class that
+  /// does not otherwise declare any constructors.
+  bool get isSyntheticDefault => flags & FlagSyntheticDefault != 0;
+
   void set isConst(bool value) {
     flags = value ? (flags | FlagConst) : (flags & ~FlagConst);
   }
 
   void set isExternal(bool value) {
     flags = value ? (flags | FlagExternal) : (flags & ~FlagExternal);
   }
 
+  void set isSyntheticDefault(bool value) {
+    flags = value
+        ? (flags | FlagSyntheticDefault)
+        : (flags & ~FlagSyntheticDefault);
+  }
+
   bool get isInstanceMember => false;
   bool get hasGetter => false;
   bool get hasSetter => false;
 
   accept(MemberVisitor v) => v.visitConstructor(this);
 
   acceptReference(MemberReferenceVisitor v) =>
       v.visitConstructorReference(this);
 
   visitChildren(Visitor v) {
@@ skipping 3484 matching lines @@
 /// typedef has not been assigned a canonical name yet.
 ///
 /// Returns `null` if the typedef is `null`.
 CanonicalName getCanonicalNameOfTypedef(Typedef typedef_) {
   if (typedef_ == null) return null;
   if (typedef_.canonicalName == null) {
     throw '$typedef_ has no canonical name';
   }
   return typedef_.canonicalName;
 }