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

pkg/kernel/lib/ast.dart

Index: pkg/kernel/lib/ast.dart
diff --git a/pkg/kernel/lib/ast.dart b/pkg/kernel/lib/ast.dart
index daa5427e83ab2e3d8af16aec6b0c292b3b547747..fa2d18086786c490994e0d73ab078909a8e60dbc 100644
--- a/pkg/kernel/lib/ast.dart
+++ b/pkg/kernel/lib/ast.dart
@@ -938,6 +938,7 @@ class Constructor extends Member {
       {Name name,
       bool isConst: false,
       bool isExternal: false,
+      bool isSyntheticDefault: false,
       List<Initializer> initializers,
       int transformerFlags: 0,
       Reference reference})
@@ -947,15 +948,21 @@ class Constructor extends Member {
     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);
   }
@@ -964,6 +971,12 @@ class Constructor extends Member {
     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;