Should infer precise-type with read-only field

Part of #464.

lang/spec.html

@@ -3741,11 +3741,12 @@ <h4>Mapping constructor</h4>
 If there is no contextually expected type, then the inherent type will be an
 exclusive-record-type-descriptor with an individual-field-descriptor for each
 specific-field; the type of each field-descriptor will be the broad type of the
-value-expr in the field. The static type of the expression in every spread-field
-will also be added to the inherent type. If there are fields specified as a
-computed-name-field, then there will also be a record-rest-descriptor
-<code>T...</code>, where <code>T</code> is the union of the broad types of the
-value-expr in all such fields.
+value-expr in the field, unless the field is read-only in which case the type of
+the field-descriptor will be the precise type. The static type of the expression
+in every spread-field will also be added to the inherent type. If there are
+fields specified as a computed-name-field, then there will also be a
+record-rest-descriptor <code>T...</code>, where <code>T</code> is the union of
+the broad types of the value-expr in all such fields.
 </p>
 <p>
 If a specific-field does not use a string-literal for the name of the field and