Support --use-kernel in memory_compiler

pkg/compiler/lib/src/js_backend/backend.dart

Index: pkg/compiler/lib/src/js_backend/backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend.dart b/pkg/compiler/lib/src/js_backend/backend.dart
index 611a64747fb4d671568b5c0105b1c11743a953dc..31f90dce47de76005565622f3905c470038a3ce5 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -911,8 +911,9 @@ class JavaScriptBackend {
    *
    * Invariant: [element] must be a declaration element.
    */
-  String getGeneratedCode(Element element) {
-    assert(element.isDeclaration, failedAt(element));
+  String getGeneratedCode(MemberEntity element) {
+    assert(!(element is MemberElement && !element.isDeclaration),
+        failedAt(element));
     return jsAst.prettyPrint(generatedCode[element], compiler.options);
   }