Fixes https://github.com/dotnet/coreclr/issues/4252 (#4384)
authorBruce Bowyer-Smyth <bbowyersmyth@live.com.au>
Fri, 22 Apr 2016 09:47:45 +0000 (19:47 +1000)
committerJan Kotas <jkotas@microsoft.com>
Fri, 22 Apr 2016 09:47:45 +0000 (02:47 -0700)
commit5fe1c0ae8eb12cf04caca09a8b4a798b87eb8408
tree3d0d5cbb30267dae16abfbc3b799247208dbb872
parentc4359c7d7b0983e62107ddf9dc1f56f21a019cd3
Fixes https://github.com/dotnet/coreclr/issues/4252 (#4384)

Change Join and Concat to access iterator.Current value once. Delegate null checking to StringBuilder.Append
src/mscorlib/src/System/String.cs