Optimize WaitGroup implementation (#21260)

`add` no longer locks a mutex, and `finish` no longer locks a mutex
except for the last task. This could meaningfully improve performance in
cases where we spawn a large number of tasks on a thread pool. This
change doesn't alter the semantics of the type, unlike the standard
library's `WaitGroup`, which also uses atomics but has to be explicitly
reset.

(For internal tracking: fixes ENG-19722)

---------

Co-authored-by: Jarred Sumner <jarred@jarredsumner.com>

scripts/sort-imports.ts

@@ -102,6 +102,10 @@ function parseDeclarations(
       continue;
     }
 
+    if (declarations.has(name)) {
+      unusedLineIndices.push(i);
+      continue;
+    }
     declarations.set(name, {
       whole: line,
       index: i,
@@ -341,20 +345,33 @@ async function processFile(filePath: string): Promise<void> {
       }
     }
     if (thisDeclaration) {
-      sortedLines[thisDeclaration.index] = DELETED_LINE;
-    }
-    if (thisDeclaration) {
-      let firstNonFileCommentLine = 0;
-      for (const line of sortedLines) {
-        if (line.startsWith("//!")) {
-          firstNonFileCommentLine++;
-        } else {
+      var onlyCommentsBeforeThis = true;
+      for (const [i, line] of sortedLines.entries()) {
+        if (i >= thisDeclaration.index) {
+          break;
+        }
+        if (line === "" || line === DELETED_LINE) {
+          continue;
+        }
+        if (!line.startsWith("//")) {
+          onlyCommentsBeforeThis = false;
           break;
         }
       }
-      const insert = [thisDeclaration.whole, ""];
-      if (firstNonFileCommentLine > 0) insert.unshift("");
-      sortedLines.splice(firstNonFileCommentLine, 0, ...insert);
+      if (!onlyCommentsBeforeThis) {
+        sortedLines[thisDeclaration.index] = DELETED_LINE;
+        let firstNonFileCommentLine = 0;
+        for (const line of sortedLines) {
+          if (line.startsWith("//!")) {
+            firstNonFileCommentLine++;
+          } else {
+            break;
+          }
+        }
+        const insert = [thisDeclaration.whole, ""];
+        if (firstNonFileCommentLine > 0) insert.unshift("");
+        sortedLines.splice(firstNonFileCommentLine, 0, ...insert);
+      }
     }
     fileContents = sortedLines.join("\n");
   }