Don't call computeSignature in the emitter.

sdk/lib/_internal/compiler/implementation/elements/modelx.dart

 // Copyright (c) 2013, 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.
 
 library elements.modelx;
 
 import 'dart:collection' show LinkedHashMap;
 
 import 'elements.dart';
 import '../../compiler.dart' as api;
@@ skipping 1401 matching lines @@
   SynthesizedConstructorElementX.forDefault(superMember, Element enclosing)
       : this(const SourceString(''), superMember, enclosing, true);
 
   Token position() => enclosingElement.position();
 
   bool get isSynthesized => true;
 
   FunctionElement get targetConstructor => superMember;
 
   FunctionSignature computeSignature(compiler) {
+    if (functionSignature != null) return functionSignature;
     if (isDefaultConstructor) {
-      return new FunctionSignatureX(
+      return functionSignature = new FunctionSignatureX(
           const Link<Element>(), const Link<Element>(), 0, 0, false,
           getEnclosingClass().thisType);
     }
     if (superMember.isErroneous()) {
-      return compiler.objectClass.localLookup(
+      return functionSignature = compiler.objectClass.localLookup(
           const SourceString('')).computeSignature(compiler);
     }
-    return superMember.computeSignature(compiler);
+    return functionSignature = superMember.computeSignature(compiler);
   }
 
   get declaration => this;
   get implementation => this;
   get defaultImplementation => this;
 }
 
 class VoidElementX extends ElementX {
   VoidElementX(Element enclosing)
       : super(const SourceString('void'), ElementKind.VOID, enclosing);
@@ skipping 747 matching lines @@
 
   MetadataAnnotation ensureResolved(Compiler compiler) {
     if (resolutionState == STATE_NOT_STARTED) {
       compiler.resolver.resolveMetadataAnnotation(this);
     }
     return this;
   }
 
   String toString() => 'MetadataAnnotation($value, $resolutionState)';
 }