code: public TypePair getTypesUnderShallowEquality(JSType that) {
      JSType commonType = getGreatestSubtype(that);
      return new TypePair(commonType, commonType);
    }
docs: Computes the subset of {@code this} and {@code that} types under shallow
  equality.
  
  @return a pair containing the restricted type of {@code this} as the first
  component and the restricted type of {@code that} as the second
  element. The returned pair is never {@code null} even though its
  components may be {@code null}.