rolling back changeset # 1401724 to see whether this is the one caused VM regression.

"When a file is replaced on disk its timestamp should be set to the most recent of its creation time and last write time. Also updating FileKey to use last-write-time, since that's what it's comment explicitly calls for." (changeset 1408149)

src/Compilers/Core/Desktop/FileKey.cs

@@ -34,7 +34,7 @@ public FileKey(string fullPath, DateTime timestamp)
         /// <exception cref="IOException"/>
         public static FileKey Create(string fullPath)
         {
-            return new FileKey(fullPath, FileUtilities.GetLastWriteTimeStamp(fullPath));
+            return new FileKey(fullPath, FileUtilities.GetFileTimeStamp(fullPath));
         }
 
         public override int GetHashCode()

src/Compilers/Core/Desktop/FileUtilities.cs

@@ -382,33 +382,13 @@ internal static void DeleteFileOnClose(string fullPath)
             }
         }
 
-        /// <exception cref="IOException"/>
-        internal static DateTime GetLastWriteTimeStamp(string fullPath)
-        {
-            Debug.Assert(PathUtilities.IsAbsolute(fullPath));
-            try
-            {
-                return File.GetLastWriteTimeUtc(fullPath);
-            }
-            catch (Exception e)
-            {
-                throw new IOException(e.Message);
-            }
-        }
-
         /// <exception cref="IOException"/>
         internal static DateTime GetFileTimeStamp(string fullPath)
         {
             Debug.Assert(PathUtilities.IsAbsolute(fullPath));
             try
             {
-                // In the case where a file is copied from one directory to the other, the last write time will stay
-                // the same but the creation time will get updated.  Return the most recent of the two times so that
-                // our caches don't run into a scenario where and older file was copied over an existing one and the
-                // cache thinks everything is up-to-date, when in fact we need to reparse the file.
-                var lastWriteTime = File.GetLastWriteTimeUtc(fullPath);
-                var creationTime = File.GetCreationTimeUtc(fullPath);
-                return creationTime > lastWriteTime ? creationTime : lastWriteTime;
+                return File.GetLastWriteTimeUtc(fullPath);
             }
             catch (Exception e)
             {

src/Scripting/Core/MetadataShadowCopyProvider.cs

@@ -357,7 +357,7 @@ private MetadataShadowCopy GetMetadataShadowCopyNoCheck(string fullPath, Metadat
             bool fault = true;
             try
             {
-                key = new FileKey(fullPath, FileUtilities.GetLastWriteTimeStamp(newCopy.Public.PrimaryModule.FullPath));
+                key = new FileKey(fullPath, FileUtilities.GetFileTimeStamp(newCopy.Public.PrimaryModule.FullPath));
                 fault = false;
             }
             finally