Fix instantiation with a Module

This commit fixes the `init` function when passed a
`WebAssembly.Module`. Upon closer reading of the [spec] we see there's
two possible return values from `WebAssembly.instantiate`. If passed a
`Module`, it will return only the `Instance`. If passed a buffer source,
though, it'll return an object with the module/instance.

The fix here is to check the result value is an `Instance`, and if so
assume the input must have been a module so it's paired up in the
output.

Closes #1418

[spec]: http://webassembly.github.io/spec/js-api/index.html#webassembly-namespace

crates/cli-support/src/js/mod.rs

@@ -903,12 +903,12 @@ impl<'a> Context<'a> {
         let ts = Self::ts_for_init_fn(mem.import.is_some());
         let js = format!(
             "\
-                function init(module_or_path{init_memory_arg}) {{
+                function init(module{init_memory_arg}) {{
                     let result;
                     const imports = {{ './{module}': __exports }};
-                    if (module_or_path instanceof URL || typeof module_or_path === 'string' || module_or_path instanceof Request) {{
+                    if (module instanceof URL || typeof module === 'string' || module instanceof Request) {{
                         {init_memory2}
-                        const response = fetch(module_or_path);
+                        const response = fetch(module);
                         if (typeof WebAssembly.instantiateStreaming === 'function') {{
                             result = WebAssembly.instantiateStreaming(response, imports)
                                 .catch(e => {{
@@ -928,9 +928,13 @@ impl<'a> Context<'a> {
                         }}
                     }} else {{
                         {init_memory1}
-                        result = WebAssembly.instantiate(module_or_path, imports)
-                            .then(instance => {{
-                                return {{ instance, module: module_or_path }};
+                        result = WebAssembly.instantiate(module, imports)
+                            .then(result => {{
+                                if (result instanceof WebAssembly.Instance) {{
+                                    return {{ instance: result, module }};
+                                }} else {{
+                                    return result;
+                                }}
                             }});
                     }}
                     return result.then(({{instance, module}}) => {{