Bump string_scanner to c637deb8d998b72a5807afbd06aba8370db725c0

Changes:
```
> git log --format="%C(auto) %h %s" 6579871..c637deb
 https://dart.googlesource.com/string_scanner.git/+/c637deb Add better support for dealing with supplemental-plane code units (#46)
 https://dart.googlesource.com/string_scanner.git/+/1259807 rev to 1.1.1 in preparation for publishing (#45)

```

Diff: https://dart.googlesource.com/string_scanner.git/+/6579871b528036767b3200b390a3ecef28e4900d~..c637deb8d998b72a5807afbd06aba8370db725c0/
Change-Id: I12e3d67ba793f2a78844097ea71660fec88bab88
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/247120
Reviewed-by: Devon Carew <devoncarew@google.com>
Auto-Submit: Nate Bosch <nbosch@google.com>
Reviewed-by: Lasse Nielsen <lrn@google.com>
Commit-Queue: Nate Bosch <nbosch@google.com>

Author: natebosch

DEPS

@@ -145,7 +145,7 @@ vars = {
   "sse_rev": "9a54f1cdd91c8d79a6bf5ef8e849a12756607453",
   "stack_trace_rev": "17f09c2c6845bb31c7c385acecce5befb8527a13",
   "stream_channel_rev": "3fa3e40c75c210d617b8b943b9b8f580e9866a89",
-  "string_scanner_rev": "6579871b528036767b3200b390a3ecef28e4900d",
+  "string_scanner_rev": "c637deb8d998b72a5807afbd06aba8370db725c0",
   "sync_http_rev": "b6bd47965694dddffb6e62fb8a6c12d17c4ae4cd",
   "term_glyph_rev": "d0f205c67ea70eea47b9f41c8440129a72a9c86e",
   "test_descriptor_rev": "5ed5d7f6bf1191592995dcb8eedbbc17df69d386",