Allow the createWasm function to use async/await where possible

The advantage if using `await` in the cases where we can is that it
avoids the generation or wrapper function for each export.

So instead of

```
var wasmExport = createWasm(); // returns empty object
...
var malloc = ((..) => (malloc = wasmExports['malloc'])()
```

We can just generate:

```
var wasmExport = createWasm(); // returns empty object
...
var malloc = wasmExports['malloc'];
```

This only currently works in MODULARIZE mode where the code is running
inside a factory function.  Otherwise it would end up using
top-level-await.

One wrinkle here is that this is not currently supported when closure is
enabled because we run closure only on the contents of the factory
function so closure ends up seeing this as a top level await when its
not.

Author: sbc100

src/preamble.js

@@ -1036,11 +1036,11 @@ function getWasmImports() {
     trueModule = null;
 #endif
 #if SHARED_MEMORY || RELOCATABLE
-    receiveInstance(result['instance'], result['module']);
+    return receiveInstance(result['instance'], result['module']);
 #else
     // TODO: Due to Closure regression https://github.com/google/closure-compiler/issues/3193, the above line no longer optimizes out down to the following line.
     // When the regression is fixed, can restore the above PTHREADS-enabled path.
-    receiveInstance(result['instance']);
+    return receiveInstance(result['instance']);
 #endif
   }
 #endif // WASM_ASYNC_COMPILATION
@@ -1076,8 +1076,7 @@ function getWasmImports() {
         // Instantiate from the module posted from the main thread.
         // We can just use sync instantiation in the worker.
         var instance = new WebAssembly.Instance(module, getWasmImports());
-        receiveInstance(instance, module);
-        resolve();
+        resolve(receiveInstance(instance, module));
       };
     });
   }
@@ -1095,16 +1094,16 @@ function getWasmImports() {
   try {
 #endif
     var result = await instantiateAsync(wasmBinary, wasmBinaryFile, info);
-    receiveInstantiationResult(result);
+    var exports = receiveInstantiationResult(result);
 #if LOAD_SOURCE_MAP
     receiveSourceMapJSON(await getSourceMapAsync());
 #endif
-    return result;
+    return exports;
 #if MODULARIZE
   } catch (e) {
     // If instantiation fails, reject the module ready promise.
     readyPromiseReject(e);
-    return;
+    return Promise.reject(e);
   }
 #endif
 #else // WASM_ASYNC_COMPILATION

test/code_size/hello_webgl2_wasm.json

@@ -1,10 +1,10 @@
 {
   "a.html": 454,
   "a.html.gz": 328,
-  "a.js": 4532,
-  "a.js.gz": 2315,
+  "a.js": 4538,
+  "a.js.gz": 2320,
   "a.wasm": 10402,
   "a.wasm.gz": 6703,
-  "total": 15388,
-  "total_gz": 9346
+  "total": 15394,
+  "total_gz": 9351
 }

test/code_size/hello_webgl2_wasm2js.json

@@ -1,8 +1,8 @@
 {
   "a.html": 346,
   "a.html.gz": 262,
-  "a.js": 22200,
-  "a.js.gz": 11583,
-  "total": 22546,
-  "total_gz": 11845
+  "a.js": 22206,
+  "a.js.gz": 11589,
+  "total": 22552,
+  "total_gz": 11851
 }

test/code_size/hello_webgl_wasm.json

@@ -1,10 +1,10 @@
 {
   "a.html": 454,
   "a.html.gz": 328,
-  "a.js": 4070,
-  "a.js.gz": 2158,
+  "a.js": 4076,
+  "a.js.gz": 2163,
   "a.wasm": 10402,
   "a.wasm.gz": 6703,
-  "total": 14926,
-  "total_gz": 9189
+  "total": 14932,
+  "total_gz": 9194
 }

test/code_size/hello_webgl_wasm2js.json

@@ -1,8 +1,8 @@
 {
   "a.html": 346,
   "a.html.gz": 262,
-  "a.js": 21726,
-  "a.js.gz": 11413,
-  "total": 22072,
-  "total_gz": 11675
+  "a.js": 21732,
+  "a.js.gz": 11419,
+  "total": 22078,
+  "total_gz": 11681
 }

test/other/codesize/test_codesize_cxx_ctors1.gzsize

@@ -1 +1 @@
-8542
+8539

test/other/codesize/test_codesize_cxx_ctors1.jssize

@@ -1 +1 @@
-20925
+20921

test/other/codesize/test_codesize_cxx_ctors2.gzsize

@@ -1 +1 @@
-8526
+8524

test/other/codesize/test_codesize_cxx_ctors2.jssize

@@ -1 +1 @@
-20893
+20889

test/other/codesize/test_codesize_cxx_except.gzsize

@@ -1 +1 @@
-9572
+9570