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diff --git a/devtools/client/styleeditor/original-source.js b/devtools/client/styleeditor/original-source.js
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+++ b/devtools/client/styleeditor/original-source.js
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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+/**
+ * An object of this type represents an original source for the style
+ * editor.  An "original" source is one that is mentioned in a source
+ * map.
+ *
+ * @param {String} url
+ *        The URL of the original source.
+ * @param {String} sourceID
+ *        The source ID of the original source, as used by the source
+ *        map service.
+ * @param {SourceMapLoader} sourceMapLoader
+ *        The source map loader; @see Toolbox.sourceMapLoader
+ */
+function OriginalSource(url, sourceId, sourceMapLoader) {
+  this.isOriginalSource = true;
+
+  this._url = url;
+  this._sourceId = sourceId;
+  this._sourceMapLoader = sourceMapLoader;
+}
+
+OriginalSource.prototype = {
+  get sourceId() {
+    return this._sourceId;
+  },
+
+  /** Get the original source's URL.  */
+  get url() {
+    return this._url;
+  },
+
+  /** Get the original source's URL.  */
+  get href() {
+    return this._url;
+  },
+
+  /**
+   * Return a promise that will resolve to the original source's full
+   * text.  The return result is actually an object with a single
+   * `string` method; this method will return the source text as a
+   * string.  This is done because the style editor elsewhere expects
+   * a long string actor.
+   */
+  getText() {
+    if (!this._sourcePromise) {
+      this._sourcePromise = this._sourceMapLoader
+        .getOriginalSourceText(this._sourceId)
+        .then(contents => {
+          // Make it look like a long string actor.
+          return {
+            string: () => contents.text,
+          };
+        });
+    }
+    return this._sourcePromise;
+  },
+
+  /**
+   * Given a source-mapped, generated style sheet, a line, and a
+   * column, return the corresponding original location in this style
+   * sheet.
+   *
+   * @param {StyleSheetResource} relatedSheet
+   *        The generated style sheet's resource
+   * @param {Number} line
+   *        Line number.
+   * @param {Number} column
+   *        Column number.
+   * @return {Location}
+   *        The original location, an object with at least
+   *        `sourceUrl`, `source`, `styleSheet`, `line`, and `column`
+   *        properties.
+   */
+  getOriginalLocation(relatedSheet, line, column) {
+    const { href, nodeHref, resourceId: sourceId } = relatedSheet;
+    const sourceUrl = href || nodeHref;
+    return this._sourceMapLoader
+      .getOriginalLocation({
+        sourceId,
+        line,
+        column,
+        sourceUrl,
+      })
+      .then(location => {
+        // Add some properties for the style editor.
+        location.source = location.sourceUrl;
+        location.styleSheet = relatedSheet;
+        return location;
+      });
+  },
+
+  // Dummy implementations, as we never emit an event.
+  on() {},
+  off() {},
+};
+
+exports.OriginalSource = OriginalSource;