Set return type of factory constructors properly.

pkg/front_end/lib/src/fasta/kernel/kernel_procedure_builder.dart

Index: pkg/front_end/lib/src/fasta/kernel/kernel_procedure_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_procedure_builder.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_procedure_builder.dart
index 210e5a11886da11fda5cb602790742f26eeabbcf..69f9ef377e0c17fa4f8c3f137bb4571d828c5a3b 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_procedure_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_procedure_builder.dart
@@ -8,6 +8,7 @@ import 'package:kernel/ast.dart'
     show
         Arguments,
         AsyncMarker,
+        Class,
         Constructor,
         ConstructorInvocation,
         DartType,
@@ -28,12 +29,14 @@ import 'package:kernel/ast.dart'
         StringLiteral,
         SuperInitializer,
         TypeParameter,
+        TypeParameterType,
         VariableDeclaration,
         VariableGet,
         VoidType,
         setParents;
 
-import 'package:kernel/type_algebra.dart' show containsTypeVariable, substitute;
+import 'package:kernel/type_algebra.dart'
+    show Substitution, containsTypeVariable, substitute;
 
 import '../errors.dart' show internalError;
 
@@ -122,6 +125,22 @@ abstract class KernelFunctionBuilder
     }
     if (!isConstructor && !isInstanceMember && parent is ClassBuilder) {
       List<TypeParameter> typeParameters = parent.target.typeParameters;
+      if (kind == ProcedureKind.Factory) {

[ahe, 2017/06/01 09:10:08]

This I have a different solution for.

+        Class class_ = parent.target;
+        if (typeParameters.isEmpty) {
+          result.returnType = class_.thisType;
+        } else {
+          // Factory constructors get their own copy of type parameters, so we
+          // have to substitute.
+          result.returnType = Substitution
+              .fromPairs(
+                  class_.typeParameters,
+                  result.typeParameters
+                      .map((p) => new TypeParameterType(p))
+                      .toList())
+              .substituteType(class_.thisType);
+        }
+      }
       if (typeParameters.isNotEmpty) {
         Map<TypeParameter, DartType> substitution;
         DartType removeTypeVariables(DartType type) {