Utilize fs.copyFileSync if available.

* Currently would only be utilized on windows
* available in node 8.5.x
* prefer Copy-On-Write if available
* good benchmarks yarnpkg/yarn#3290

Although node-copy-file-sync module exists, it uses more modern JS features. Which this library cannot yet support (without major version bump)

index.js

@@ -123,6 +123,13 @@ function symlink(_srcPath, _destPath) {
 // Fix for https://github.com/broccolijs/broccoli-merge-trees/issues/42
 var WINDOWS_PREFIX = "\\\\?\\";
 
+function copyFileSync(srcPath, destPath, stat) {
+  options.fs.writeFileSync(destPath, options.fs.readFileSync(srcPath), {
+    flag: 'wx',
+    mode: stat.mode
+  });
+}
+
 function symlinkWindows(srcPath, destPath) {
   var stat = options.fs.lstatSync(srcPath);
   var isDir = stat.isDirectory();
@@ -143,7 +150,11 @@ function symlinkWindows(srcPath, destPath) {
     if (isDir) {
       options.fs.symlinkSync(srcPath, destPath, 'junction');
     } else {
-      options.fs.writeFileSync(destPath, options.fs.readFileSync(srcPath), { flag: 'wx', mode: stat.mode });
+      if (options.fs.copyFileSync) {
+        options.fs.copyFileSync(srcPath, destPath);
+      } else {
+        copyFileSync(srcPath, destPath, stat);
+      }
       options.fs.utimesSync(destPath, stat.atime, stat.mtime);
     }
   }

tests/index.js

@@ -122,6 +122,58 @@ describe('symlink-or-copy', function() {
       assert.equal(utimesSyncCount, 1);
     });
 
+    it('windows falls back to copyFileSync if available for file', function() {
+      var count = 0
+      var lstatSyncCount = 0
+      var isDirectoryCount = 0
+      var readFileSyncCount = 0
+      var writeFileSyncCount = 0
+      var utimesSyncCount = 0
+      var copyFileSync = 0
+      symLinkOrCopy.setOptions({
+        isWindows: true,
+        canSymLink: false,
+        fs: {
+          lstatSync: function() {
+            lstatSyncCount++;
+            return {
+              isSymbolicLink: function() {
+                return true;
+              },
+              isDirectory: function() {
+                isDirectoryCount++;
+                return false;
+              }
+            };
+          },
+          readFileSync: function() {
+            readFileSyncCount++;
+            return 'foo';
+          },
+          writeFileSync: function() {
+            writeFileSyncCount++;
+            return 'foo';
+          },
+          copyFileSync: function() {
+            copyFileSync++;
+            return 'foo';
+          },
+          realpathSync: function() {count++;},
+          symlinkSync: function() {count++;},
+          utimesSync: function() {utimesSyncCount++;}
+        }
+      });
+
+      symLinkOrCopy.sync('foo', 'bar');
+      assert.equal(count, 1);
+      assert.equal(lstatSyncCount, 2);
+      assert.equal(isDirectoryCount, 2);
+      assert.equal(writeFileSyncCount, 0);
+      assert.equal(copyFileSync, 1);
+      assert.equal(readFileSyncCount, 0);
+      assert.equal(utimesSyncCount, 1);
+    });
+
     it('windows symlinks when has permission', function() {
       var count = 0;
       symLinkOrCopy.setOptions({