First pass at asynchronous input loading in dart2js.

tests/compiler/dart2js/list_tracer_length_test.dart

Index: tests/compiler/dart2js/list_tracer_length_test.dart
diff --git a/tests/compiler/dart2js/list_tracer_length_test.dart b/tests/compiler/dart2js/list_tracer_length_test.dart
index 2a48ef50fe658751f4f74bb9d47b2abc3ed2e184..e853764fb89e1f6ff129ac1f2f8c75dd526d4e4b 100644
--- a/tests/compiler/dart2js/list_tracer_length_test.dart
+++ b/tests/compiler/dart2js/list_tracer_length_test.dart
@@ -63,8 +63,9 @@ main() {
 """;
 
 void checkRangeError(String test, {bool hasRangeError}) {
-  String generated = compileAll(test);
-  Expect.equals(hasRangeError, generated.contains('ioore'));
+  compileAll(test).then((generated) {
+    Expect.equals(hasRangeError, generated.contains('ioore'));
+  });
 }
 
 main() {