Make sourcemap plainobject for use w / valueToNode

When the sourcemaps were switched to use "@ampproject/remapping", the type
of the inputSourceMap that gets passed around changed from being a plain js
object to a `class SourceMap` fromthe remapping package. This causes issues
with the @babel/types valueToNode function because that function is defined
to throw for objects that aren't plain-objects.

In practice I was seeing this error after upgrading babel when running code-
coverage, and I saw a similar error reported here: vuejs/vue-jest#450 before
deciding to try to track the error down myself.

packages/babel-core/src/transformation/file/merge-map.ts

@@ -30,7 +30,13 @@ export default function mergeSourceMap(
   if (typeof inputMap.sourceRoot === "string") {
     result.sourceRoot = inputMap.sourceRoot;
   }
-  return result;
+
+  // remapping returns a SourceMap class type, but this breaks code downstream in
+  // @babel/traverse and @babel/types that relies on data being plain objects.
+  // When it encounters the sourcemap type it outputs a "don't know how to turn
+  // this value into a node" error. As a result, we are converting the merged
+  // sourcemap to a plain js object.
+  return { ...result };
 }
 
 function rootless(map: SourceMap): SourceMap {

packages/babel-core/test/merge-map.js

@@ -0,0 +1,28 @@
+import _mergeSourceMap from "../lib/transformation/file/merge-map.js";
+const mergeSourceMap = _mergeSourceMap.default;
+
+const inputMap = {
+  file: "file.js",
+  mappings: [],
+  names: [],
+  sources: ["file.ts"],
+  version: 3,
+};
+
+const outputMap = {
+  file: "file.transpiled.js",
+  mappings: [],
+  names: [],
+  sources: ["file.js"],
+  version: 3,
+};
+
+describe("merge-map", () => {
+  it("returns a plain js object", () => {
+    const map = mergeSourceMap(inputMap, outputMap, "file.transpiled.js");
+    const proto = Object.getPrototypeOf(map) ?? Object.getPrototypeOf({});
+    expect(typeof map).toBe("object");
+    expect(Object.prototype.toString.call(map)).toBe("object [object]");
+    expect(Object.getPrototypeOf(proto)).toBeNull();
+  });
+});