support separate instance lookup function

Author: guybedford

document/js-api/index.bs

@@ -80,6 +80,7 @@ urlPrefix: https://tc39.github.io/ecma262/; spec: ECMASCRIPT
         text: Cyclic Module Record; url: cyclic-module-record
         text: GetMethod; url: sec-getmethod
         text: ToBigInt64; url: #sec-tobigint64
+        text: Module Namespace exotic object; url: #sec-module-namespace-exotic-objects
     type: abstract-op
         text: CreateDataPropertyOrThrow; url: sec-createdatapropertyorthrow
         text: CreateMethodProperty; url: sec-createmethodproperty
@@ -318,6 +319,8 @@ namespace WebAssembly {
 
     Promise<Instance> instantiate(
         Module moduleObject, optional object importObject);
+
+    Instance moduleInstance(ModuleNamespace namespace);
 };
 </pre>
 
@@ -534,6 +537,14 @@ The verification of WebAssembly type requirements is deferred to the
     1. [=asynchronously instantiate a WebAssembly module|Asynchronously instantiate the WebAssembly module=] |moduleObject| importing |importObject|, and return the result.
 </div>
 
+<div algorithm>
+  The <dfn method for="WebAssembly">moduleInstance(|namespace|)</dfn> method, when invoked, performs the following steps:
+    1. If |namespace| is not a [=is a Module Namespace exotic object=], [=throw=] a {{TypeError}} exception.
+    1. If |namespace|.\[[Module]] is not a [=WebAssembly Module Record=], [=throw=] a {{TypeError}} exception.
+    1. Let |module| be |namespace|.\[[Module]].
+    1. Return |module|.\[[Instance]].
+</div>
+
 Note: A follow-on streaming API is documented in the <a href="https://webassembly.github.io/spec/web-api/index.html">WebAssembly Web API</a>.
 
 <h3 id="modules">Modules</h3>