Handle signature types and fields in kernel_impact

pkg/compiler/lib/src/elements/common.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.
 
 /// Mixins that implement convenience methods on [Element] subclasses.
 
 library elements.common;
 
 import '../common/names.dart' show Identifiers, Names, Uris;
 import '../core_types.dart' show CoreClasses;
@@ skipping 476 matching lines @@
     }
     return null;
   }
 
   void forEachBackendMember(void f(Element member)) {
     backendMembers.forEach(f);
   }
 }
 
 abstract class FunctionSignatureCommon implements FunctionSignature {
+  DartType get returnType => type.returnType;
+
   void forEachRequiredParameter(void function(Element parameter)) {
     requiredParameters.forEach(function);
   }
 
   void forEachOptionalParameter(void function(Element parameter)) {
     optionalParameters.forEach(function);
   }
 
   void forEachParameter(void function(Element parameter)) {
     forEachRequiredParameter(function);
@@ skipping 128 matching lines @@
   @override
   bool get isBoolFromEnvironmentConstructor {
     return fromEnvironmentState == _FromEnvironmentState.BOOL;
   }
 
   @override
   bool get isStringFromEnvironmentConstructor {
     return fromEnvironmentState == _FromEnvironmentState.STRING;
   }
 }