Make dart2js unittests async.

tests/compiler/dart2js/union_type_test.dart

Index: tests/compiler/dart2js/union_type_test.dart
diff --git a/tests/compiler/dart2js/union_type_test.dart b/tests/compiler/dart2js/union_type_test.dart
index 984880f535ecf73fe32bbbf96dc06ebf71b98af6..a519f0d112c2719bd25484e80446524a218b122d 100644
--- a/tests/compiler/dart2js/union_type_test.dart
+++ b/tests/compiler/dart2js/union_type_test.dart
@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import "package:expect/expect.dart";
+import "package:async_helper/async_helper.dart";
 import "compiler_helper.dart";
 import "parser_helper.dart";
 
@@ -10,15 +11,16 @@ import "../../../sdk/lib/_internal/compiler/implementation/types/types.dart";
 import "../../../sdk/lib/_internal/compiler/implementation/dart_types.dart";
 
 main() {
-  MockCompiler compiler = new MockCompiler();
-  compiler.intClass.ensureResolved(compiler);
-  compiler.stringClass.ensureResolved(compiler);
+  asyncTest(() => MockCompiler.create((MockCompiler compiler) {
+    compiler.intClass.ensureResolved(compiler);
+    compiler.stringClass.ensureResolved(compiler);
 
-  FlatTypeMask mask1 =
-      new FlatTypeMask.exact(compiler.intClass);
-  FlatTypeMask mask2 =
-      new FlatTypeMask.exact(compiler.stringClass);
-  UnionTypeMask union1 = mask1.nonNullable().union(mask2, compiler);
-  UnionTypeMask union2 = mask2.nonNullable().union(mask1, compiler);
-  Expect.equals(union1, union2);
+    FlatTypeMask mask1 =
+        new FlatTypeMask.exact(compiler.intClass);
+    FlatTypeMask mask2 =
+        new FlatTypeMask.exact(compiler.stringClass);
+    UnionTypeMask union1 = mask1.nonNullable().union(mask2, compiler);
+    UnionTypeMask union2 = mask2.nonNullable().union(mask1, compiler);
+    Expect.equals(union1, union2);
+  }));
 }