[dart2js] Ensure setter Invocation names end with "=".

This is the same approach DDC and the VM take. Longer term, it may make
sense to move this logic into the CFE.

Change-Id: I9118747f187ca7bfa7eabe74d45e69a8c4cf240c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/155823
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Commit-Queue: Mayank Patke <fishythefish@google.com>

Author: fishythefish

pkg/compiler/lib/src/kernel/dart2js_target.dart

@@ -127,7 +127,7 @@ class Dart2jsTarget extends Target {
       name = name.substring(4);
     } else if (name.startsWith('set:')) {
       kind = invocationMirrorSetterKind;
-      name = name.substring(4);
+      name = name.substring(4) + '=';
     } else {
       kind = invocationMirrorMethodKind;
     }