code: public TypeMirror inferConditionalExpressionType(TypeMirror trueType, TypeMirror falseType) {
      return isAssignable(trueType, falseType) ? falseType : trueType;
    }
docs: TODO(user): See jls-5.6.2 and jls-15.25.
  @param trueType the type of the true expression
  @param falseType the type of the false expression
  @return the inferred type of the conditional expression