Use async reads if possible

Change-Id: Ib7ae51e5f79e10797376cc3e4ca1a5bf6708fa30
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/185307
Auto-Submit: Gary Roumanis <grouma@google.com>
Commit-Queue: Sigmund Cherem <sigmund@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>

pkg/dev_compiler/lib/src/kernel/expression_compiler_worker.dart

@@ -463,8 +463,8 @@ class ExpressionCompilerWorker {
 
   Future<Component> _loadComponent(Uri uri) async {
     var file = _processedOptions.fileSystem.entityForUri(uri);
-    if (await file.exists()) {
-      var bytes = await file.readAsBytes();
+    if (await file.existsAsyncIfPossible()) {
+      var bytes = await file.readAsBytesAsyncIfPossible();
       var component = _processedOptions.loadComponent(bytes, _sdkComponent.root,
           alwaysCreateNewNamedNodes: true);
       return component;