Use WebAssembly.compileStreaming API if available (#5606)

This API is a post-MVP addition
(http://webassembly.org/docs/web/#additional-web-embedding-api) that does
streaming compilation (i.e compiles the wasm binary as it downloads), resulting
in faster instantiation in the non-cached case.

src/preamble.js

@@ -2318,19 +2318,37 @@ function integrateWasmJS() {
     // later), so we save Module and check it later.
     var trueModule = Module;
 #endif
-    getBinaryPromise().then(function(binary) {
-      return WebAssembly.instantiate(binary, info)
-    }).then(function(output) {
+    function receiveInstantiatedSource(output) {
+      // 'output' is a WebAssemblyInstantiatedSource object which has both the module and instance.
       // receiveInstance() will swap in the exports (to Module.asm) so they can be called
 #if ASSERTIONS
       assert(Module === trueModule, 'the Module object should not be replaced during async compilation - perhaps the order of HTML elements is wrong?');
       trueModule = null;
 #endif
       receiveInstance(output['instance']);
-    }).catch(function(reason) {
-      Module['printErr']('failed to asynchronously prepare wasm: ' + reason);
-      abort(reason);
-    });
+    }
+    function instantiateArrayBuffer(receiver) {
+      getBinaryPromise().then(function(binary) {
+        return WebAssembly.instantiate(binary, info);
+      }).then(receiver).catch(function(reason) {
+        Module['printErr']('failed to asynchronously prepare wasm: ' + reason);
+        abort(reason);
+      });
+    }
+    // Prefer streaming instantiation if available.
+    if (!Module['wasmBinary'] && typeof WebAssembly.instantiateStreaming === 'function') {
+      WebAssembly.instantiateStreaming(fetch(wasmBinaryFile, { credentials: 'same-origin' }), info)
+        .then(receiveInstantiatedSource)
+        .catch(function(reason) {
+          // We expect the most common failure cause to be a bad MIME type for the binary,
+          // in which case falling back to ArrayBuffer instantiation should work.
+          Module['printErr']('wasm streaming compile failed: ' + reason);
+          Module['printErr']('falling back to ArrayBuffer instantiation');
+          instantiateArrayBuffer(receiveInstantiatedSource);
+        });
+    } else {
+      instantiateArrayBuffer(receiveInstantiatedSource);
+    }
     return {}; // no exports yet; we'll fill them in later
 #else
     var instance;

tests/runner.py

@@ -746,6 +746,8 @@ def do_GET(self):
     def log_request(code=0, size=0):
       # don't log; too noisy
       pass
+
+  SimpleHTTPServer.SimpleHTTPRequestHandler.extensions_map['.wasm'] = 'application/wasm'
   os.chdir(dir)
   httpd = BaseHTTPServer.HTTPServer(('localhost', 8888), TestServerHandler)
   httpd.serve_forever() # test runner will kill us