Basic test benchmark for WebAssembly integration.

This is not nearly as efficient as a native build of the transform
binary, due to bugs such as http://crbug.com/853685 and
golang/go#27462, but serves as a starting
point for further investigation.

There have been a few optimizations already:
 - Keeping one long-lived process open to amortize the bootstrapping
   cost across requests.
 - Batching the "requests" to the wasm code via Promise.all. (This can
   be seen by adding a log statement just before the call to exports.run
   in wasm_exec.js.)

Another potential problem to investigate the cost of is the lack of
cross-heap GC.

cmd/transform_wasm/main.go

@@ -0,0 +1,43 @@
+// Copyright 2018 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+/*
+Binary wrapper for the transformer lib, to be run via main.js.
+*/
+package main
+
+import (
+	"syscall/js"
+
+	rpb "github.com/ampproject/amppackager/transformer/request"
+	t "github.com/ampproject/amppackager/transformer"
+)
+
+func transform(args []js.Value /* url, html, cb(htmlout) */) {
+	r := &rpb.Request{Html: args[1].String(), DocumentUrl: args[0].String(), Config: rpb.Request_DEFAULT}
+	o, _, err := t.Process(r)
+	if err != nil {
+		panic(err)
+	}
+	args[2].Invoke(o + "\n")
+}
+
+func main() {
+	cb := js.NewCallback(transform)
+	defer cb.Release()
+	done := make(chan struct{})
+	donecb := js.NewCallback(func(args []js.Value) { done <- struct{}{} })
+	js.Global().Get("begin").Invoke(cb, donecb)
+	<-done
+}

cmd/transform_wasm/main.js

@@ -0,0 +1,57 @@
+// Copyright 2018 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Bootstrapper for transform_wasm. Transforms all html files in
+// ${TESTDIR:-/tmp/amps}.
+//
+// To use:
+//   GOOS=js GOARCH=wasm go build -o transform.wasm ./cmd/transform_wasm/ &&
+//   node --max-old-space-size=4096 cmd/transform_wasm/main.js transform.wasm
+
+// TODO(twifkak): Investigate slowdown over time.
+// TODO(twifkak): Investigate memory usage growth.
+
+const assert = require('assert');
+const { join } = require('path');
+const { spawnSync } = require('child_process');
+
+function getRecursive(dir = process.env.TESTDIR || '/tmp/amps') {
+  return [].concat(...fs.readdirSync(dir, {withFileTypes: true}).map((dirent) =>
+      dirent.isDirectory() ? getRecursive(join(dir, dirent.name)) : join(dir, dirent.name)));
+}
+
+global.begin = async function(transform, done) {
+  const htmlPaths = getRecursive().filter((file) => file.endsWith('.html'));
+  let num = 0;
+  let outs = [];
+  for (path of htmlPaths) {
+    const html = fs.readFileSync(path);
+    outs.push(new Promise((resolve) => {
+      transform('https://example.com/', html, (amphtml) => {
+          if (++num % 100 == 0) console.log('num = ', num);
+          assert(amphtml.length > 1000);  // "Minimum valid AMP" is larger than 1K.
+          resolve(amphtml);
+      });
+    }));
+  }
+  console.log('Pushed all %d thunks.', htmlPaths.length);
+  const start = process.hrtime.bigint();
+  await Promise.all(outs);
+  const total = process.hrtime.bigint() - start;
+  console.log(`Took ${total} nanoseconds, or ${Number(total) / htmlPaths.length / 1000000} millis per doc.`);
+  done();
+}
+
+const goroot = process.env.GOROOT || spawnSync('go', ['env', 'GOROOT']).stdout.toString().trim();
+require(join(goroot, 'misc/wasm/wasm_exec.js'));