More extensive error handling for 'unresolved' cases.

test/kernel/regression/super_operator.dart.txt

 library;
 import self as self;
 import "dart:core" as core;
 
 class A extends core::Object {
   constructor •() → self::A
     : super core::Object::•();
   operator +(core::String s) → dynamic
     return null;
   operator [](dynamic i) → dynamic
     return null;
   operator []=(dynamic i, dynamic val) → dynamic {}
 }
 class B extends self::A {
   constructor •() → self::B
     : super self::A::•();
   operator +(core::String s) → dynamic
     return super.self::A::+("${s}${s}");
   operator [](dynamic i) → dynamic
     return super.self::A::[](i);
   operator []=(dynamic i, dynamic val) → dynamic
     return let final dynamic #t1 = let final dynamic #t2 = i in let final dynamic #t3 = i = #t2.+(1) in #t2 in super.self::A::[]=(#t1, super.self::A::[](#t1).+(val));
 }
 class Autobianchi extends core::Object {
   constructor •() → self::Autobianchi
     : super core::Object::•();
   method g() → dynamic
-    return invalid-expression;
+    return let final dynamic #t4 = 0 in throw core::_unresolvedSuperMethodError(this, #[], <dynamic>[#t4], <dynamic, dynamic>{}, null);
 }