Handle signature types and fields in kernel_impact

pkg/compiler/lib/src/elements/common.dart

Index: pkg/compiler/lib/src/elements/common.dart
diff --git a/pkg/compiler/lib/src/elements/common.dart b/pkg/compiler/lib/src/elements/common.dart
index 2e99eff1d87df21a2ec428a4076f33336ad58242..3e430d2ae90d3e38eb46b0d17060c285539de532 100644
--- a/pkg/compiler/lib/src/elements/common.dart
+++ b/pkg/compiler/lib/src/elements/common.dart
@@ -494,6 +494,8 @@ abstract class ClassElementCommon implements ClassElement {
 }
 
 abstract class FunctionSignatureCommon implements FunctionSignature {
+  DartType get returnType => type.returnType;
+
   void forEachRequiredParameter(void function(Element parameter)) {
     requiredParameters.forEach(function);
   }