First pass at asynchronous input loading in dart2js.

tests/compiler/dart2js/private_test.dart

Index: tests/compiler/dart2js/private_test.dart
diff --git a/tests/compiler/dart2js/private_test.dart b/tests/compiler/dart2js/private_test.dart
index 3390c9f2ae8336c7cd39aa82c164216c1ad35434..457eee428871a97530f91d84740ae743d5d97c42 100644
--- a/tests/compiler/dart2js/private_test.dart
+++ b/tests/compiler/dart2js/private_test.dart
@@ -68,8 +68,9 @@ void analyze(String text, [expectedWarnings]) {
                   ''';
   Uri uri = Uri.parse('src:public');
   compiler.registerSource(uri, source);
-  compiler.runCompiler(uri);
-  compareWarningKinds(text, expectedWarnings, compiler.warnings);
+  compiler.runCompiler(uri).then((_) {
+    compareWarningKinds(text, expectedWarnings, compiler.warnings);
+  });
 }
 
 void main() {