Fix #62883 String buffer clear()

Closes https://github.com/dart-lang/sdk/pull/62931

GitOrigin-RevId: 00a2f30d0906758176575a615d57fdb6c517e23a
Change-Id: I636770b919f3f2358136d05fc373cbd53e88e4ce
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/489040
Reviewed-by: Slava Egorov <vegorov@google.com>
Commit-Queue: Slava Egorov <vegorov@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>

sdk/lib/_internal/vm_shared/lib/string_buffer_patch.dart

@@ -123,7 +123,8 @@ class StringBuffer {
   @patch
   void clear() {
     _parts = null;
-    _partsCodeUnits = _bufferPosition = _bufferCodeUnitMagnitude = 0;
+    _partsCodeUnits = _bufferPosition = _bufferCodeUnitMagnitude =
+        _partsCompactionIndex = _partsCodeUnitsSinceCompaction = 0;
   }
 
   /** Returns the contents of buffer as a string. */
@@ -175,6 +176,7 @@ class StringBuffer {
     } else {
       localParts.add(str);
       int partsSinceCompaction = localParts.length - _partsCompactionIndex;
+      assert(partsSinceCompaction > 0);
       if (partsSinceCompaction == _PARTS_TO_COMPACT) {
         _compact();
       }

tests/corelib/string_buffer_clear_compaction_regression_test.dart

@@ -0,0 +1,30 @@
+// Copyright (c) 2026, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+// VMOptions=--enable-asserts
+
+import "package:expect/expect.dart";
+
+const int _iterations = 10000;
+final String _expected = List.generate(
+  _iterations,
+  (int i) => i.isEven ? "foo " : "bar ",
+).join();
+
+void writeFooBar(StringBuffer buffer) {
+  for (int i = 0; i < _iterations; i++) {
+    buffer.write(i.isEven ? "foo" : "bar");
+    buffer.write(" ");
+  }
+}
+
+void main() {
+  final buffer = StringBuffer();
+
+  writeFooBar(buffer);
+  buffer.clear();
+  writeFooBar(buffer);
+
+  Expect.equals(_expected.length, buffer.length);
+  Expect.equals(_expected, buffer.toString());
+}