Don't call computeSignature in the emitter.

sdk/lib/_internal/compiler/implementation/resolution/members.dart

Index: sdk/lib/_internal/compiler/implementation/resolution/members.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/resolution/members.dart	(revision 26119)
+++ sdk/lib/_internal/compiler/implementation/resolution/members.dart	(working copy)
@@ -341,6 +341,10 @@
       }
       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);