First pass at asynchronous input loading in dart2js.

tests/compiler/dart2js/package_root_test.dart

Index: tests/compiler/dart2js/package_root_test.dart
diff --git a/tests/compiler/dart2js/package_root_test.dart b/tests/compiler/dart2js/package_root_test.dart
index 62c106d693c93ab9db9c0214251a91e7171f6923..350a5ba9560435b18f28259c1891f07f583d30eb 100644
--- a/tests/compiler/dart2js/package_root_test.dart
+++ b/tests/compiler/dart2js/package_root_test.dart
@@ -52,18 +52,19 @@ void runCompiler(Uri main) {
     return new NullSink('$name.$extension');
   }
 
-  Compiler compiler = new Compiler(provider.readStringFromUri,
+  Compiler compiler = new Compiler(provider,
                                    outputProvider,
                                    diagnosticHandler,
                                    libraryRoot,
                                    null,
                                    []);
 
-  compiler.run(main);
-  Expect.equals(1, errors.length);
-  Expect.equals('Error: Cannot resolve "package:foo/foo.dart". '
-                'Package root has not been set.',
-                errors[0]);
+  compiler.run(main).then((_) {
+    Expect.equals(1, errors.length);
+    Expect.equals('Error: Cannot resolve "package:foo/foo.dart". '
+                  'Package root has not been set.',
+                  errors[0]);
+  });
 }
 
 void main() {