Make super.foo= erroneous if they are not assignable.

pkg/compiler/lib/src/warnings.dart

Index: pkg/compiler/lib/src/warnings.dart
diff --git a/pkg/compiler/lib/src/warnings.dart b/pkg/compiler/lib/src/warnings.dart
index a9cad07c86704eb3d68b3dc82df16ed1d4dbfa4f..2c8a7f7e6a80e6875a46e3e93901d7df4b6b6e70 100644
--- a/pkg/compiler/lib/src/warnings.dart
+++ b/pkg/compiler/lib/src/warnings.dart
@@ -130,6 +130,9 @@ class MessageKind {
   static const MessageKind SETTER_NOT_FOUND = const MessageKind(
       "No setter named '#{memberName}' in class '#{className}'.");
 
+  static const MessageKind SETTER_NOT_FOUND_IN_SUPER = const MessageKind(
+      "No setter named '#{name}' in superclass of '#{className}'.");
+
   static const MessageKind GETTER_NOT_FOUND = const MessageKind(
       "No getter named '#{memberName}' in class '#{className}'.");
 
@@ -1126,6 +1129,10 @@ main() => new C();"""]);
   static const MessageKind ASSIGNING_METHOD = const MessageKind(
       "Cannot assign a value to a method.");
 
+  static const MessageKind ASSIGNING_METHOD_IN_SUPER = const MessageKind(
+      "Cannot assign a value to method '#{name}' "
+      "in superclass '#{superclassName}'.");
+
   static const MessageKind ASSIGNING_TYPE = const MessageKind(
       "Cannot assign a value to a type.");