[Augmentations] No error in the analyzer if augmented is torn-off inside of an augmenting operator

[sgrekhov]

According to the specification

When augmenting an operator, augmented must be followed by the operator.

But see, for example https://github.com/dart-lang/co19/blob/master/LanguageFeatures/Augmentation-libraries/augmented_expression_A05_t01_lib1.dart

const augmented = "Augmented constant, should not be used";

augment class C {
  augment int operator +(Object other) {
    var f = augmented; // No expected error in the analyzer
//          ^^^^^^^^^
// [analyzer] unspecified
// [cfe] unspecified
    return 0;
  }
}