[dart2js] Polyfill Object.assign()
[dart2js] Polyfill Object.assign()

Change-Id: I4ae5172c2e39106be3609d5c6a569d176fdc0066
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/107581
Commit-Queue: Mayank Patke <e0355b78204b67a55a952941c68c8f1b40235938@google.com>
Auto-Submit: Mayank Patke <e0355b78204b67a55a952941c68c8f1b40235938@google.com>
Reviewed-by: Stephen Adams <60cb5b73b7e0cb7dcda53302da4fab6ecfc7fa8b@google.com>

dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk