Support linting the spec

package.json

@@ -5,9 +5,11 @@
   "description": "The Source Map specification",
   "scripts": {
     "prebuild-only": "npm run clean && mkdir out  && cp -R img out",
-    "build-only": "ecmarkup --verbose --load-biblio @tc39/ecma262-biblio spec.emu out/index.html",
-    "build": "npm run build-only",
+    "build-only": "npm run ecmarkup -- --verbose --load-biblio @tc39/ecma262-biblio spec.emu out/index.html",
+    "build-loose": "npm run build-only",
+    "build": "npm run build-only -- --lint-spec",
     "clean": "rm -rf out",
+    "ecmarkup": "node ./scripts/ecmarkup.js",
     "format": "emu-format --write spec.emu",
     "test": "exit 0",
     "watch": "npm run build-only -- --watch"

scripts/ecmarkup.js

@@ -0,0 +1,53 @@
+function wrapWarnFunction(originalWarn) {
+  return (err) => {
+    if (
+      err.message.includes("Completion Record") ||
+      err.message.includes("an abrupt completion")
+    ) {
+      // Ignore errors about completion records
+      return;
+    }
+
+    let match;
+    if (match = /^could not find definition for (\w+)$/.exec(err.message)) {
+      switch (match[1]) {
+        // This is correctly <dfn>ed as a reference from an external
+        // specification. However, ecmarkup only allows "calling" AOs and not
+        // <dfn>ed terms.
+        case "module_decode":
+        // Ecmarkup mistakenly detects HTTP in "an HTTP(S) scheme" as an AO
+        // call.
+        case "HTTP":
+          return;
+      }
+    }
+
+    originalWarn(err);
+  };
+}
+
+Object.defineProperty(Object.prototype, "warn", {
+  enumerable: false,
+  configurable: true,
+  get: () => undefined,
+  set(value) {
+    // Trying to detect this object:
+    // https://github.com/tc39/ecmarkup/blob/734baa009be2bdbab29c14a9e52ed3da1e26caa3/src/cli.ts#L104
+    if (
+      Object.hasOwn(this, "multipage") &&
+      Object.hasOwn(this, "outfile") &&
+      Object.hasOwn(this, "extraBiblios") &&
+      Object.hasOwn(this, "lintSpec")
+    ) {
+      value = wrapWarnFunction(value);
+    }
+    Object.defineProperty(this, "warn", {
+      enumerable: true,
+      configurable: true,
+      writable: true,
+      value,
+    });
+  },
+});
+
+require("ecmarkup/bin/ecmarkup.js");