DevTools: Cache the original content on UISourceCode

third_party/WebKit/Source/devtools/front_end/snippets/ScriptSnippetModel.js

Index: third_party/WebKit/Source/devtools/front_end/snippets/ScriptSnippetModel.js
diff --git a/third_party/WebKit/Source/devtools/front_end/snippets/ScriptSnippetModel.js b/third_party/WebKit/Source/devtools/front_end/snippets/ScriptSnippetModel.js
index 0dc66a264cc229478caee2209668dd844227d723..29f937f8b10fcaae9f5c139c831c4cee574dad73 100644
--- a/third_party/WebKit/Source/devtools/front_end/snippets/ScriptSnippetModel.js
+++ b/third_party/WebKit/Source/devtools/front_end/snippets/ScriptSnippetModel.js
@@ -554,6 +554,8 @@ Snippets.SnippetsProject = class extends Bindings.ContentProviderBasedProject {
   constructor(workspace, model) {
     super(workspace, 'snippets:', Workspace.projectTypes.Snippets, '', false /* isServiceProject */);
     this._model = model;
+    /** @type {!WeakMap<!Workspace.UISourceCode, !Promise<?string>>} */
+    this._originalContent = new WeakMap();
   }
 
   /**
@@ -572,16 +574,30 @@ Snippets.SnippetsProject = class extends Bindings.ContentProviderBasedProject {
   canSetFileContent() {
     return true;
   }
+  /**
+   * @override
+   * @param {!Workspace.UISourceCode} uiSourceCode
+   * @return {!Promise<?string>}
+   */
+  requestOriginalFileContent(uiSourceCode) {
+    if (this._originalContent.has(uiSourceCode))
+      return this._originalContent.get(uiSourceCode);
+    var promise = super.requestOriginalFileContent(uiSourceCode);
+    this._originalContent.set(uiSourceCode, promise);
+    return promise;
+  }
 
   /**
    * @override
    * @param {!Workspace.UISourceCode} uiSourceCode
    * @param {string} newContent
-   * @param {function(?string)} callback
+   * @param {function()} callback
    */
   setFileContent(uiSourceCode, newContent, callback) {
-    this._model._setScriptSnippetContent(uiSourceCode.url(), newContent);
-    callback('');
+    this.requestOriginalFileContent(uiSourceCode).then(() => {
+      this._model._setScriptSnippetContent(uiSourceCode.url(), newContent);
+      callback();
+    });
   }
 
   /**