First pass at asynchronous input loading in dart2js.

tests/compiler/dart2js/unneeded_part_js_test.dart

Index: tests/compiler/dart2js/unneeded_part_js_test.dart
diff --git a/tests/compiler/dart2js/unneeded_part_js_test.dart b/tests/compiler/dart2js/unneeded_part_js_test.dart
index cebcd91051675dea564858d945111454dbb744e8..ace2f424bcbcb2c5a320077facc4944f44daf467 100644
--- a/tests/compiler/dart2js/unneeded_part_js_test.dart
+++ b/tests/compiler/dart2js/unneeded_part_js_test.dart
@@ -41,8 +41,9 @@ main() {
                                    libraryRoot,
                                    packageRoot,
                                    []);
-  compiler.run(Uri.parse('memory:main.dart'));
-  Expect.isFalse(compiler.compilationFailed);
+  compiler.run(Uri.parse('memory:main.dart')).then((_) {
+    Expect.isFalse(compiler.compilationFailed);
+  });
 }
 
 const Map MEMORY_SOURCE_FILES = const {