Don't call computeSignature in the emitter.

sdk/lib/_internal/compiler/implementation/resolution/members.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.
 
 part of resolution;
 
 abstract class TreeElements {
   Element get currentElement;
   Set<Node> get superUses;
 
@@ skipping 323 matching lines @@
       bool isConstructor =
           identical(element.kind, ElementKind.GENERATIVE_CONSTRUCTOR);
       TreeElements elements =
           compiler.enqueuer.resolution.getCachedElements(element);
       if (elements != null) {
         assert(isConstructor);
         return elements;
       }
       if (element.isSynthesized) {
         Element target = element.targetConstructor;
+        // Ensure the signature of the synthesized element is
+        // resolved. This is the only place where the resolver is
+        // seeing this element.
+        element.computeSignature(compiler);
         if (!target.isErroneous()) {
           compiler.enqueuer.resolution.registerStaticUse(
               element.targetConstructor);
         }
         return new TreeElementMapping(element);
       }
       if (element.isPatched) {
         checkMatchingPatchSignatures(element, element.patch);
         element = element.patch;
       }
@@ skipping 3732 matching lines @@
     return e;
   }
 
   /// Assumed to be called by [resolveRedirectingFactory].
   Element visitReturn(Return node) {
     Node expression = node.expression;
     return finishConstructorReference(visit(expression),
                                       expression, expression);
   }
 }