[dart2wasm, standalone] Rename JSStringImpl to EmbedderStringImpl

The standalone target used `JSStringImpl` as the name for its string
implementation even though JavaScript isn't involved in that at all.
This was to simplify parts of the compiler which can then refer to both
classes with the same name.

Changing this in the compiler is not that complicated however, so it
makes sense to align the string implementation name with the embedder
terminology we also use for other host imports.

TEST=pkg/dart2wasm/test/ir_tests/standalone.{dart,wat}

Change-Id: I1e112c8a72bb43a7edfa73ff7205d353edc7403a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/504581
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>
Reviewed-by: Slava Egorov <vegorov@google.com>

pkg/kernel/lib/target/targets.dart

@@ -588,10 +588,6 @@ abstract class Target {
   Class? concreteDoubleLiteralClass(CoreTypes coreTypes, double value) => null;
   Class? concreteStringLiteralClass(CoreTypes coreTypes, String value) => null;
 
-  /// When a comparison `x == <literal>` is true, whether we can assume the
-  /// class of `x` to be `concreteStringLiteralClass(<literal>)`.
-  bool get canInferStringClassAfterEqualityComparison => true;
-
   Class? concreteAsyncResultClass(CoreTypes coreTypes) => null;
   Class? concreteSyncStarResultClass(CoreTypes coreTypes) => null;