Implement the local JS snippets RFC

This commit is an implementation of [RFC 6] which enables crates to
inline local JS snippets into the final output artifact of
`wasm-bindgen`. This is accompanied with a few minor breaking changes
which are intended to be relatively minor in practice:

* The `module` attribute disallows paths starting with `./` and `../`.
  It requires paths starting with `/` to actually exist on the filesystem.
* The `--browser` flag no longer emits bundler-compatible code, but
  rather emits an ES module that can be natively loaded into a browser.

Otherwise be sure to check out [the RFC][RFC 6] for more details, and
otherwise this should implement at least the MVP version of the RFC!
Notably at this time JS snippets with `--nodejs` or `--no-modules` are
not supported and will unconditionally generate an error.

[RFC 6]: https://github.com/rustwasm/rfcs/pull/6

Closes #1311

crates/cli/src/bin/wasm-bindgen-test-runner/main.rs

@@ -107,7 +107,8 @@ fn rmain() -> Result<(), Error> {
     shell.status("Executing bindgen...");
     let mut b = Bindgen::new();
     b.debug(debug)
-        .nodejs(node)
+        .nodejs(node)?
+        .browser(!node)?
         .input_module(module, wasm)
         .keep_debug(false)
         .emit_start(false)

crates/cli/src/bin/wasm-bindgen-test-runner/server.rs

@@ -5,7 +5,6 @@ use std::path::Path;
 
 use failure::{format_err, Error, ResultExt};
 use rouille::{Request, Response, Server};
-use wasm_bindgen_cli_support::wasm2es6js::Config;
 
 pub fn spawn(
     addr: &SocketAddr,
@@ -23,9 +22,9 @@ pub fn spawn(
             __wbgtest_console_log,
             __wbgtest_console_info,
             __wbgtest_console_warn,
-            __wbgtest_console_error
+            __wbgtest_console_error,
+            default as init,
         }} from './{0}';
-        import * as wasm from './{0}_bg';
 
         // Now that we've gotten to the point where JS is executing, update our
         // status text as at this point we should be asynchronously fetching the
@@ -33,9 +32,7 @@ pub fn spawn(
         document.getElementById('output').textContent = "Loading wasm module...";
 
         async function main(test) {{
-            // this is a facet of using wasm2es6js, a hack until browsers have
-            // native ESM support for wasm modules.
-            await wasm.booted;
+            const wasm = await init('./{0}_bg.wasm');
 
             const cx = new Context();
             window.on_console_debug = __wbgtest_console_debug;
@@ -65,25 +62,6 @@ pub fn spawn(
     let js_path = tmpdir.join("run.js");
     fs::write(&js_path, js_to_execute).context("failed to write JS file")?;
 
-    // No browser today supports a wasm file as ES modules natively, so we need
-    // to shim it. Use `wasm2es6js` here to fetch an appropriate URL and look
-    // like an ES module with the wasm module under the hood.
-    //
-    // TODO: don't reparse the wasm module here, should pass the
-    //       `Module struct` directly from the output of
-    //       `wasm-bindgen` previously here and avoid unnecessary
-    //       parsing.
-    let wasm_name = format!("{}_bg.wasm", module);
-    let wasm = fs::read(tmpdir.join(&wasm_name))?;
-    let output = Config::new()
-        .fetch(Some(format!("/{}", wasm_name)))
-        .generate(&wasm)?;
-    let (js, wasm) = output.js_and_wasm()?;
-    let wasm = wasm.unwrap();
-    fs::write(tmpdir.join(format!("{}_bg.js", module)), js).context("failed to write JS file")?;
-    fs::write(tmpdir.join(format!("{}_bg.wasm", module)), wasm)
-        .context("failed to write wasm file")?;
-
     // For now, always run forever on this port. We may update this later!
     let tmpdir = tmpdir.to_path_buf();
     let srv = Server::new(addr, move |request| {

crates/cli/src/bin/wasm-bindgen.rs

@@ -88,9 +88,9 @@ fn rmain(args: &Args) -> Result<(), Error> {
 
     let mut b = Bindgen::new();
     b.input_path(input)
-        .nodejs(args.flag_nodejs)
-        .browser(args.flag_browser)
-        .no_modules(args.flag_no_modules)
+        .nodejs(args.flag_nodejs)?
+        .browser(args.flag_browser)?
+        .no_modules(args.flag_no_modules)?
         .debug(args.flag_debug)
         .demangle(!args.flag_no_demangle)
         .keep_debug(args.flag_keep_debug)
@@ -98,7 +98,7 @@ fn rmain(args: &Args) -> Result<(), Error> {
         .remove_producers_section(args.flag_remove_producers_section)
         .typescript(typescript);
     if let Some(ref name) = args.flag_no_modules_global {
-        b.no_modules_global(name);
+        b.no_modules_global(name)?;
     }
     if let Some(ref name) = args.flag_out_name {
         b.out_name(name);