Specialize instance calls when the call receiver is the method receiver and the method class has a …

runtime/lib/string_patch.dart

Index: runtime/lib/string_patch.dart
diff --git a/runtime/lib/string_patch.dart b/runtime/lib/string_patch.dart
index cdb94bea9506fef61da25ab5952f71d9239d5347..f7aa83c5522b3d31143e1f73e4e3ecb0225d9f29 100644
--- a/runtime/lib/string_patch.dart
+++ b/runtime/lib/string_patch.dart
@@ -47,7 +47,7 @@ patch class String {
  * [_StringBase] contains common methods used by concrete String
  * implementations, e.g., _OneByteString.
  */
-class _StringBase {
+abstract class _StringBase {
   // Constants used by replaceAll encoding of string slices between matches.
   // A string slice (start+length) is encoded in a single Smi to save memory
   // overhead in the common case.