Turn error into warning for non-static member access through the class.

sdk/lib/_internal/compiler/implementation/elements/modelx.dart

Index: sdk/lib/_internal/compiler/implementation/elements/modelx.dart
diff --git a/sdk/lib/_internal/compiler/implementation/elements/modelx.dart b/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
index 9715c62d71eb0037b12a59c8b471c28ab377f814..00bcbd2c85d1fb1bc2ae8656db828d8bd6708311 100644
--- a/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
+++ b/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
@@ -1015,6 +1015,10 @@ class AbstractFieldElementX extends ElementX implements AbstractFieldElement {
           setter.modifiers.flags | Modifiers.FLAG_ABSTRACT);
     }
   }
+
+  bool isInstanceMember() {
+    return isMember() && !modifiers.isStatic();
+  }
 }
 
 // TODO(johnniwinther): [FunctionSignature] should be merged with