mutable JSON module

Closes https://github.com/tc39/proposal-import-attributes/issues/54

README.md

@@ -106,6 +106,7 @@ In the context of the [WebAssembly/ESM integration proposal](https://github.com/
 JSON modules are required to be supported when invoked using the `if { type: "json" }` syntax, with common semantics in all JavaScript implementations for this syntax.
 
 JSON modules' semantics are those of a single default export which is the entire parsed JSON document.
+The imported object should be mutable and a single instance should exist per Realm [#54](https://github.com/tc39/proposal-import-attributes/issues/54).
 
 Each JavaScript host is expected to provide a secondary way of checking whether a module is a JSON module. For example, on the Web, the MIME type would be checked to be a JSON MIME type. In "local" desktop/server/embedded environments, the file extension may be checked (possibly after symlinks are followed). The `type: "json"` is indicated at the import site, rather than *only* through that other mechanism in order to prevent the privilege escalation issue noted in the opening section.
 

spec.html

@@ -143,6 +143,10 @@ <h1>Runtime Semantics: HostResolveImportedModule ( _referencingScriptOrModule_,
         <emu-note type=editor>
           <p>The above text implies that hosts *must* support JSON modules imported with `type: "json"` (if it completes normally), but it doesn't prohibit hosts from supporting JSON modules imported with no type specified. Some environments (for example, web browsers) are expected to require `if { type: "json" }`, and environments which want to restrict themselves to a compatible subset would do so as well.</p>
         </emu-note>
+
+        <emu-note type=editor>
+          <p>The imported object should be mutable and a single instance should exist per Realm.</p>
+        </emu-note>
       </emu-clause>
 
       <emu-clause id="sec-hostimportmoduledynamically" aoid="HostImportModuleDynamically">