First pass at asynchronous input loading in dart2js.

tests/compiler/dart2js/emit_const_fields_test.dart

Index: tests/compiler/dart2js/emit_const_fields_test.dart
diff --git a/tests/compiler/dart2js/emit_const_fields_test.dart b/tests/compiler/dart2js/emit_const_fields_test.dart
index d539d896236f5d7ad1afdbb06b8b0d77e8baad8f..14385d7d7a7b6010bc67eb6deed66a9f4e20bae5 100644
--- a/tests/compiler/dart2js/emit_const_fields_test.dart
+++ b/tests/compiler/dart2js/emit_const_fields_test.dart
@@ -4,6 +4,7 @@
 // Test that unused static consts are not emitted.
 
 import "package:expect/expect.dart";
+import "package:async_helper/async_helper.dart";
 import 'compiler_helper.dart';
 
 const String TEST_GUIDE = r"""
@@ -18,8 +19,9 @@ main() {
 """;
 
 main() {
-  String generated = compileAll(TEST_GUIDE);
-  Expect.isTrue(generated.contains("42"));
-  Expect.isFalse(generated.contains("TITLE"));
+  asyncTest(() => compileAll(TEST_GUIDE).then((generated) {
+    Expect.isTrue(generated.contains("42"));
+    Expect.isFalse(generated.contains("TITLE"));
+  }));
 }