Make .enclosingClass always return the declaration

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

Index: pkg/compiler/lib/src/resolution/constructors.dart
diff --git a/pkg/compiler/lib/src/resolution/constructors.dart b/pkg/compiler/lib/src/resolution/constructors.dart
index bd127d0ef3a7abb2a970fd9a4aaa046db7446246..87c8cc72e0253561ea1dc62fdbd1ffab1e64193d 100644
--- a/pkg/compiler/lib/src/resolution/constructors.dart
+++ b/pkg/compiler/lib/src/resolution/constructors.dart
@@ -98,7 +98,10 @@ class InitializerResolver {
     Element target;
     FieldElement field;
     if (isFieldInitializer(init)) {
-      target = constructor.enclosingClass.lookupLocalMember(name);
+      // Use [enclosingElement] instead of [enclosingClass] to ensure lookup in
+      // patch class when necessary.
+      ClassElement cls = constructor.enclosingElement;
+      target = cls.lookupLocalMember(name);
       if (target == null) {
         reporter.reportErrorMessage(
             selector, MessageKind.CANNOT_RESOLVE, {'name': name});