Add package-config tests where test is running in Isolate.spawn-isolates.

tests/standalone/packages_file_test.dart

Index: tests/standalone/packages_file_test.dart
diff --git a/tests/standalone/packages_file_test.dart b/tests/standalone/packages_file_test.dart
index 0b59c29b96e5d2cb3a956361c719c8e040c70b39..719d3d7fb3b37ccfa7b1dad5aab232706b9cf02b 100644
--- a/tests/standalone/packages_file_test.dart
+++ b/tests/standalone/packages_file_test.dart
@@ -245,7 +245,7 @@ void createConfigurations() {
       });
     }
 
-    String fixPaths(Map dirs) {
+    void fixPaths(Map dirs) {
       for (var name in dirs.keys) {
         var value = dirs[name];
         if (value is Map) {
@@ -934,7 +934,9 @@ class Configuration {
       config: config ?? this.config,
       mainFile: mainFile ??
           ((main == null) ? this.mainFile : "${this.mainPath}$main.dart"),
-      args: args ?? ([]..addAll(newArgs ?? const [])..addAll(this.args)),
+      args:
+          args ?? (<String>[]..addAll(newArgs ?? const <String>[])
+                             ..addAll(this.args)),
       expect: expect == null
           ? this.expect
           : new Map.from(this.expect)..addAll(expect ?? const {}));