Change how unresolved super sends are handled.

pkg/front_end/testcases/rasta/mixin_library.dart.strong.expect

+library test.mixin_library;
+import self as self;
+import "dart:core" as core;
+
+class Mixin<T extends core::Object> extends core::Object {
+  field dynamic x = self::f();
+  field dynamic y = null;
+  field dynamic z = null;
+  field self::Mixin::T t = null;
+  constructor •() → void
+    : super core::Object::•()
+    ;
+  method foo() → dynamic
+    return super.foo().+(self::f());
+  method g(self::Mixin::T a) → self::Mixin::T
+    return null;
+  method h() → dynamic
+    return self::V();
+  method l() → dynamic
+    return self::_private();
+  method _privateMethod() → dynamic
+    return 49;
+  method publicMethod() → dynamic
+    return this.{self::Mixin::_privateMethod}();
+}
+static method f() → dynamic
+  return 2;
+static method V() → dynamic
+  return 87;
+static method _private() → dynamic
+  return 117;
+static method foo(dynamic m) → dynamic
+  return m._privateMethod();