[Devtools] Lazily build reverse mappings

third_party/WebKit/Source/devtools/front_end/bindings/BlackboxManager.js

Index: third_party/WebKit/Source/devtools/front_end/bindings/BlackboxManager.js
diff --git a/third_party/WebKit/Source/devtools/front_end/bindings/BlackboxManager.js b/third_party/WebKit/Source/devtools/front_end/bindings/BlackboxManager.js
index e24342f8afa8a1bbff2232faf7b4b0ed7845044b..d2c3e8ff4fcb04e7a16d49266f456ba9621dc1bd 100644
--- a/third_party/WebKit/Source/devtools/front_end/bindings/BlackboxManager.js
+++ b/third_party/WebKit/Source/devtools/front_end/bindings/BlackboxManager.js
@@ -149,14 +149,14 @@ WebInspector.BlackboxManager.prototype = {
         if (!previousScriptState)
             return Promise.resolve();
 
-        var mappings = sourceMap.mappings().slice();
-        mappings.sort(mappingComparator);
-
+        var hasBlackboxedMappings = sourceMap.sourceURLs().some((url) => this.isBlackboxedURL(url));
+        var mappings = hasBlackboxedMappings ? sourceMap.mappings().slice() : [];
         if (!mappings.length) {

[lushnikov, 2016/10/01 04:23:50]

how's a sourceMap with empty mappings differs from a "no sourcemap" case?

@kozyatinskiy: could you please give some insights on this?

[kozy, 2016/10/01 04:40:21]

There are no big difference between empty source map and no source map. This
cases can be merge into no mappings case.

lgtm for blackboxmanager.

             if (previousScriptState.length > 0)
                 return this._setScriptState(script, []);
             return Promise.resolve();
         }
+        mappings.sort(mappingComparator);
 
         var currentBlackboxed = false;
         var isBlackboxed = false;