Disallow `null` in spanFromSpannable.

pkg/compiler/lib/src/resolution/constructors.dart

 // Copyright (c) 2015, 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 dart2js.resolution.constructors;
 
 import '../common.dart';
 import '../common/resolution.dart' show Resolution;
 import '../constants/constructors.dart'
     show
@@ skipping 327 matching lines @@
         }
       }
       if (element.isInitializingFormal) {
         VariableDefinitions variableDefinitions = parameterNodes.head;
         Node parameterNode = variableDefinitions.definitions.nodes.head;
         InitializingFormalElementX initializingFormal = element;
         FieldElement field = initializingFormal.fieldElement;
         if (!field.isMalformed) {
           registry.registerStaticUse(new StaticUse.fieldInit(field));
         }
-        checkForDuplicateInitializers(field, element.initializer);
+        checkForDuplicateInitializers(field, parameterNode);
         visitor.defineLocalVariable(parameterNode, initializingFormal);
         visitor.addToScope(initializingFormal);
         if (isConst) {
           if (element.isNamed) {
             fieldInitializers[field] = new NamedArgumentReference(element.name);
           } else {
             int index = element.functionDeclaration.parameters.indexOf(element);
             fieldInitializers[field] = new PositionalArgumentReference(index);
           }
         } else {
@@ skipping 520 matching lines @@
     // constructors.
     return null;
   }
   // TODO(johnniwinther): Use [Name] for lookup.
   ConstructorElement constructor = cls.lookupConstructor(constructorName);
   if (constructor != null) {
     constructor = constructor.declaration;
   }
   return constructor;
 }