Use entities in CommonElements interface.

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

 // Copyright (c) 2014, 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.signatures;
 
 import '../common.dart';
 import '../common/resolution.dart';
 import '../elements/resolution_types.dart';
 import '../elements/elements.dart';
@@ skipping 372 matching lines @@
         // Use async marker to determine the return type of function
         // expressions.
         FunctionElement function = element;
         asyncMarker = function.asyncMarker;
       }
       switch (asyncMarker) {
         case AsyncMarker.SYNC:
           returnType = visitor.resolveReturnType(returnNode);
           break;
         case AsyncMarker.SYNC_STAR:
-          returnType = resolution.commonElements.iterableType();
+          ResolutionInterfaceType iterableType =
+              resolution.commonElements.iterableType();
+          returnType = iterableType;
           break;
         case AsyncMarker.ASYNC:
-          returnType = resolution.commonElements.futureType();
+          ResolutionInterfaceType futureType =
+              resolution.commonElements.futureType();
+          returnType = futureType;
           break;
         case AsyncMarker.ASYNC_STAR:
-          returnType = resolution.commonElements.streamType();
+          ResolutionInterfaceType streamType =
+              resolution.commonElements.streamType();
+          returnType = streamType;
           break;
       }
     }
 
     if (element.isSetter &&
         (requiredParameterCount != 1 || visitor.optionalParameterCount != 0)) {
       // If there are no formal parameters, we already reported an error above.
       if (formalParameters != null) {
         reporter.reportErrorMessage(
             formalParameters, MessageKind.ILLEGAL_SETTER_FORMALS);
@@ skipping 75 matching lines @@
 /// variables of the function signature itself when its signature is analyzed.
 class FunctionSignatureBuildingScope extends TypeVariablesScope {
   @override
   final List<ResolutionDartType> typeVariables;
 
   FunctionSignatureBuildingScope(Scope parent, this.typeVariables)
       : super(parent);
 
   String toString() => 'FunctionSignatureBuildingScope($typeVariables)';
 }