A generative consutructor on an interface can default to a factory constructor.

lib/compiler/implementation/ssa/builder.dart

 // Copyright (c) 2012, 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.
 
 class Interceptors {
   Compiler compiler;
   Interceptors(Compiler this.compiler);
 
   SourceString mapOperatorToMethodName(Operator op) {
     String name = op.source.stringValue;
@@ skipping 2135 matching lines @@
         element.kind == ElementKind.GENERATIVE_CONSTRUCTOR) {
       bool succeeded = addStaticSendArgumentsToList(selector, node.arguments,
                                                     element, inputs);
       if (!succeeded) {
         // TODO(ngeoffray): Match the VM behavior and throw an
         // exception at runtime.
         compiler.cancel('Unimplemented non-matching static call', node: node);
       }
       HType type = HType.UNKNOWN;
       Element originalElement = elements[node];
-      if (originalElement.isGenerativeConstructor()) {
-        ClassElement cls = originalElement.enclosingElement;
-        if (cls === compiler.listClass) {
-          type = HType.MUTABLE_ARRAY;
-        } else {
-          type = new HNonPrimitiveType(cls.type);
-        }
+      if (originalElement.isGenerativeConstructor()
+          && originalElement.enclosingElement === compiler.listClass) {
+        type = HType.MUTABLE_ARRAY;
+      } else if (element.isGenerativeConstructor()) {
+        ClassElement cls = element.enclosingElement;
+        type = new HNonPrimitiveType(cls.type);
       }
       push(new HInvokeStatic(selector, inputs, type));
     } else {
       if (element.kind == ElementKind.GETTER) {
         target = new HInvokeStatic(Selector.GETTER, inputs);
         add(target);
         inputs = <HInstruction>[target];
       }
       addDynamicSendArgumentsToList(node, inputs);
       push(new HInvokeClosure(selector, inputs));
@@ skipping 936 matching lines @@
                                         false,
                                         <HInstruction>[target, input]));
     return builder.pop();
   }
 
   HInstruction result() {
     flushLiterals();
     return prefix;
   }
 }