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Dart's type promotion rules aren't sophisticated enough to understand that (a is num && b is num) || (a is String && b is String) means that it is safe to evaluate (a + b), so we have to split it into two separate cases. TBR=mfairhurst@google.com,scheglov@google.com Review-Url: https://codereview.chromium.org/2605583002 .