Handle forwarding constructors

Handle serialization of forwarding constructors that are defined by
forwarding constructors:

  class A {}
  class B {}
  class C {}
  class D = A with B, C;

The default constructor of D is a forwarding constructor defined by the
default constructor of its super type A+B, which itself is a forwarding
constructor. Thus, the defining constructor might itself be synthesized
and therefore serialized indirectly.

R=het@google.com

Review URL: https://codereview.chromium.org/2105183003 .

pkg/compiler/lib/src/serialization/element_serialization.dart

@@ -423,7 +423,8 @@ class ConstructorSerializer implements ElementSerializer {
       SerializedElementKind kind) {
     SerializerUtil.serializeParentRelation(element, encoder);
     if (kind == SerializedElementKind.FORWARDING_CONSTRUCTOR) {
-      encoder.setElement(Key.ELEMENT, element.definingConstructor);
+      serializeElementReference(element.enclosingClass, Key.ELEMENT, Key.NAME,
+          encoder, element.definingConstructor);
     } else {
       SerializerUtil.serializeMetadata(element, encoder);
       encoder.setType(Key.TYPE, element.type);
@@ -773,8 +774,10 @@ class ElementDeserializer {
       case SerializedElementKind.REDIRECTING_FACTORY_CONSTRUCTOR:
         return new RedirectingFactoryConstructorElementZ(decoder);
       case SerializedElementKind.FORWARDING_CONSTRUCTOR:
-        return new ForwardingConstructorElementZ(
-            decoder.getElement(Key.CLASS), decoder.getElement(Key.ELEMENT));
+        ClassElement cls = decoder.getElement(Key.CLASS);
+        Element definingConstructor =
+            deserializeElementReference(cls, Key.ELEMENT, Key.NAME, decoder);
+        return new ForwardingConstructorElementZ(cls, definingConstructor);
       case SerializedElementKind.TOPLEVEL_FUNCTION:
         return new TopLevelFunctionElementZ(decoder);
       case SerializedElementKind.STATIC_FUNCTION:

pkg/compiler/lib/src/serialization/serialization_util.dart

@@ -506,10 +506,19 @@ Element deserializeElementReference(
     if (elementName == null) {
       return null;
     }
-    assert(invariant(NO_LOCATION_SPANNABLE, context.isConstructor,
-        message: "Unexpected reference of forwarding constructor "
-            "'${elementName}' from $context."));
-    ClassElement superclass = context.enclosingClass.superclass;
+    ClassElement cls;
+    if (context is ClassElement) {
+      assert(invariant(NO_LOCATION_SPANNABLE, context.isNamedMixinApplication,
+          message: "Unexpected reference of forwarding constructor "
+              "'${elementName}' from $context."));
+      cls = context;
+    } else {
+      assert(invariant(NO_LOCATION_SPANNABLE, context.isConstructor,
+          message: "Unexpected reference of forwarding constructor "
+              "'${elementName}' from $context."));
+      cls = context.enclosingClass;
+    }
+    ClassElement superclass = cls.superclass;
     element = superclass.lookupConstructor(elementName);
     assert(invariant(NO_LOCATION_SPANNABLE, element != null,
         message: "Unresolved reference of forwarding constructor "

tests/compiler/dart2js/serialization/test_data.dart

@@ -452,6 +452,21 @@ class A {
   final field1;
   const A(a) : this.field1 = a + 1.0;
 }
+''',
+  }),
+
+  const Test('Forwarding constructor defined by forwarding constructor', const {
+    'main.dart': '''
+import 'a.dart';
+
+main() => new C();
+''',
+  }, preserializedSourceFiles: const {
+    'a.dart': '''
+class A {}
+class B {}
+class C {}
+class D = A with B, C;
 ''',
   }),
 ];