[dart2wasm] Move JSStringImpl to dart:_string

This moves `JSStringImpl` class from `dart:_js_types` to `dart:_string`.

This allows implementing a common base class with unchecked operations
to all string classes as internal methods (so users won't be able to
call them via `dyanmic`), and extension methods to call these unchecked
methods in libraries like `dart:convert`.

Uses of these methods are introduced in
https://dart-review.googlesource.com/c/sdk/+/372443.

Change-Id: Ie4cfe778654c42d62bc4a90391fe349fa783a42c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/372442
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Ömer Ağacan <omersa@google.com>

https://dart.googlesource.com/sdk/+/a954aa0200171a6c9a6b66a81a86c3f6864e86a2

DEPS

diff --git a/DEPS b/DEPS
index 9c36739..bd3f3e1 100644
--- a/DEPS
+++ b/DEPS
@@ -282,7 +282,7 @@
 
   'engine/src/flutter': Var('flutter_git') + '/mirrors/engine' + '@' + 'ed2ff31f70ea922b33bc0a625523f4a38b5a8b60',
 
-  'engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + 'a616e736b3847e82fd07eb7dca192c597e60fa38',
+  'engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + 'a954aa0200171a6c9a6b66a81a86c3f6864e86a2',
 
   'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '64dd1ca9bc32fbc767a3f63e4c9c5ae2fc355380',
 

commits.json

diff --git a/commits.json b/commits.json
index 0291507..ee1302b 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "engine/src/flutter":"ed2ff31f70ea922b33bc0a625523f4a38b5a8b60",
-  "engine/src/flutter/third_party/dart":"a616e736b3847e82fd07eb7dca192c597e60fa38",
+  "engine/src/flutter/third_party/dart":"a954aa0200171a6c9a6b66a81a86c3f6864e86a2",
   "flutter":"64dd1ca9bc32fbc767a3f63e4c9c5ae2fc355380"
 }
\ No newline at end of file