improve source-map generation (#3782)

- emit singleton segments to mark generated code from input

test/input/issue-3040/expect.js

@@ -1,2 +1,41 @@
-function _toConsumableArray(arr){if(Array.isArray(arr)){for(var i=0,arr2=Array(arr.length);i<arr.length;i++){arr2[i]=arr[i]}return arr2}else{return Array.from(arr)}}var _require=require("bar"),foo=_require.foo;var _require2=require("world"),hello=_require2.hello;foo.x.apply(foo,_toConsumableArray(foo.y(hello.z)));
-//# sourceMappingURL=data:application/json;charset=utf-8;base64,eyJ2ZXJzaW9uIjozLCJzb3VyY2VzIjpbImlucHV0Mi5qcyJdLCJzb3VyY2VzQ29udGVudCI6WyJjb25zdCB7Zm9vfSA9IHJlcXVpcmUoXCJiYXJcIik7XG5jb25zdCB7aGVsbG99ID0gcmVxdWlyZShcIndvcmxkXCIpO1xuXG5mb28ueCguLi5mb28ueShoZWxsby56KSk7XG4iXSwibmFtZXMiOlsicmVxdWlyZSIsImFyciJdLCJtYXBwaW5ncyI6IjBKQUFjQSxLQUFRQyJ9
+"use strict";
+
+function _toConsumableArray(arr) {
+    return _arrayWithoutHoles(arr) || _iterableToArray(arr) || _unsupportedIterableToArray(arr) || _nonIterableSpread();
+}
+
+function _nonIterableSpread() {
+    throw new TypeError("Invalid attempt to spread non-iterable instance.\nIn order to be iterable, non-array objects must have a [Symbol.iterator]() method.");
+}
+
+function _unsupportedIterableToArray(o, minLen) {
+    if (!o) return;
+    if (typeof o === "string") return _arrayLikeToArray(o, minLen);
+    var n = Object.prototype.toString.call(o).slice(8, -1);
+    if (n === "Object" && o.constructor) n = o.constructor.name;
+    if (n === "Map" || n === "Set") return Array.from(n);
+    if (n === "Arguments" || /^(?:Ui|I)nt(?:8|16|32)(?:Clamped)?Array$/.test(n)) return _arrayLikeToArray(o, minLen);
+}
+
+function _iterableToArray(iter) {
+    if (typeof Symbol !== "undefined" && Symbol.iterator in Object(iter)) return Array.from(iter);
+}
+
+function _arrayWithoutHoles(arr) {
+    if (Array.isArray(arr)) return _arrayLikeToArray(arr);
+}
+
+function _arrayLikeToArray(arr, len) {
+    if (len == null || len > arr.length) len = arr.length;
+    for (var i = 0, arr2 = new Array(len); i < len; i++) {
+        arr2[i] = arr[i];
+    }
+    return arr2;
+}
+
+var _require = require("bar"), foo = _require.foo;
+
+var _require2 = require("world"), hello = _require2.hello;
+
+foo.x.apply(foo, _toConsumableArray(foo.y(hello.z)));
+//# sourceMappingURL=data:application/json;charset=utf-8;base64,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