[ddc] Cleanup false assumption from subtype check

FutureOr<A> <: FutureOr<B> iff A <: B was proven to not always be true.
Changing this to simply an optimization and no longer "if and only if".

Change-Id: I46f228933df65a86c2c525a48f11b18397b7c48c
Fixes: #38818
Fixes: #42326
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/151325
Reviewed-by: Mark Zhou <[email protected]>
Commit-Queue: Nicholas Shahan <[email protected]>

Author: nshahan

sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/types.dart

@@ -1415,10 +1415,10 @@ bool _isSubtype(t1, t2, @notNull bool strictMode) => JS<bool>('!', '''(() => {
     let t1TypeArg = ${getGenericArgs(t1)}[0];
     if (${_isFutureOr(t2)}) {
       let t2TypeArg = ${getGenericArgs(t2)}[0];
-      // FutureOr<A> <: FutureOr<B> iff A <: B
-      // TODO(nshahan): Proven to not actually be true and needs cleanup.
-      // https://github.com/dart-lang/sdk/issues/38818
-      return $_isSubtype(t1TypeArg, t2TypeArg, $strictMode);
+      // FutureOr<A> <: FutureOr<B> if A <: B
+      if ($_isSubtype(t1TypeArg, t2TypeArg, $strictMode)) {
+        return true;
+      }
     }
 
     // given t1 is Future<A> | A, then:

sdk_nnbd/lib/_internal/js_dev_runtime/private/ddc_runtime/types.dart

@@ -1413,10 +1413,10 @@ bool _isSubtype(t1, t2, @notNull bool strictMode) => JS<bool>('!', '''(() => {
     let t1TypeArg = ${getGenericArgs(t1)}[0];
     if (${_isFutureOr(t2)}) {
       let t2TypeArg = ${getGenericArgs(t2)}[0];
-      // FutureOr<A> <: FutureOr<B> iff A <: B
-      // TODO(nshahan): Proven to not actually be true and needs cleanup.
-      // https://github.com/dart-lang/sdk/issues/38818
-      return $_isSubtype(t1TypeArg, t2TypeArg, $strictMode);
+      // FutureOr<A> <: FutureOr<B> if A <: B
+      if ($_isSubtype(t1TypeArg, t2TypeArg, $strictMode)) {
+        return true;
+      }
     }
 
     // given t1 is Future<A> | A, then: