Replace site-shared submodule with git dependencies (#6898)

The `inject_dartpad.dart.js` script needs to be inlined now that
site-shared is not a submodule. However, this is temporary as we plan to
replace this with a new Dart API directly from a Jaspr component. The
API is added in dart-lang/site-shared#247.

Author: parlough

.gitmodules

@@ -30,7 +30,10 @@ dependencies:
 
 dev_dependencies:
   analysis_defaults:
-    path: ../site-shared/pkgs/analysis_defaults
+    git:
+      url: https://github.com/dart-lang/site-shared
+      path: pkgs/analysis_defaults
+      ref: 88aa84df953e67b7595b1e214b717f26d81ed538
   build_runner: ^2.8.0
   build_web_compilers: ^4.2.3
   jaspr_builder: ^0.21.1