Use entities in CommonElements interface.

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

Index: pkg/compiler/lib/src/resolution/signatures.dart
diff --git a/pkg/compiler/lib/src/resolution/signatures.dart b/pkg/compiler/lib/src/resolution/signatures.dart
index ae39454ca961667060499877c13bbffcf1d8e08f..07f3bbaf9407b7761bd184a3d20b4ad4d7d5a0ef 100644
--- a/pkg/compiler/lib/src/resolution/signatures.dart
+++ b/pkg/compiler/lib/src/resolution/signatures.dart
@@ -390,13 +390,19 @@ class SignatureResolver extends MappingVisitor<FormalElementX> {
           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;
       }
     }