resolve symbolic links before checking temp dir location (#1605)

On mac `Directory.systemTemp` starts with `/var` which is a symlink to `/private/var`, but the temp dir created by the ScratchSpace has the full resolved path (`/private/var/...`).

test/barback/dartdevc/scratch_space_test.dart

@@ -39,7 +39,9 @@ void main() {
     });
 
     test('Can create and delete a scratch space', () async {
-      expect(p.isWithin(Directory.systemTemp.path, scratchSpace.tempDir.path),
+      expect(
+          p.isWithin(Directory.systemTemp.resolveSymbolicLinksSync(),
+              scratchSpace.tempDir.resolveSymbolicLinksSync()),
           isTrue);
 
       for (var id in allAssets.keys) {