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 aa1e641dbdbeb9dc454641db8bc933bd41a9086a..afe22a80e41ff18386ffaf3ffb13c361d59e304f 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 {