First pass at asynchronous input loading in dart2js.

tests/compiler/dart2js/field_codegen_test.dart

Index: tests/compiler/dart2js/field_codegen_test.dart
diff --git a/tests/compiler/dart2js/field_codegen_test.dart b/tests/compiler/dart2js/field_codegen_test.dart
index d9a8aec6543750968bd980ce3c29bcb063f4c160..d2b597818ca5baeb92437ce593d1f98d5fa8702d 100644
--- a/tests/compiler/dart2js/field_codegen_test.dart
+++ b/tests/compiler/dart2js/field_codegen_test.dart
@@ -20,8 +20,11 @@ main() { return x; }
 """;
 
 main() {
-  String generated = compileAll(TEST_NULL0);
-  Expect.isTrue(generated.contains("null"));
-  generated = compileAll(TEST_NULL1);
-  Expect.isTrue(generated.contains("null"));
+  compileAll(TEST_NULL0).then((generated) {
+    Expect.isTrue(generated.contains("null"));
+  });
+
+  compileAll(TEST_NULL1).then((generated) {
+    Expect.isTrue(generated.contains("null"));
+  });
 }