First pass at asynchronous input loading in dart2js.

tests/compiler/dart2js/reexport_handled_test.dart

Index: tests/compiler/dart2js/reexport_handled_test.dart
diff --git a/tests/compiler/dart2js/reexport_handled_test.dart b/tests/compiler/dart2js/reexport_handled_test.dart
index d309fcd94741957dbea46366faf812c8fd242a33..64897206caaca32b483e57adc37521b7be2915a9 100644
--- a/tests/compiler/dart2js/reexport_handled_test.dart
+++ b/tests/compiler/dart2js/reexport_handled_test.dart
@@ -28,19 +28,21 @@ void main() {
   compiler.registerSource(reexportingLibraryUri, REEXPORTING_LIBRARY_SOURCE);
 
   // Load exporting library before the reexporting library.
-  var exportingLibrary = compiler.libraryLoader.loadLibrary(
-      exportingLibraryUri, null, exportingLibraryUri);
-  Expect.isTrue(exportingLibrary.exportsHandled);
-  var foo = findInExports(exportingLibrary, 'foo');
-  Expect.isNotNull(foo);
-  Expect.isTrue(foo.isField());
-
-  // Load reexporting library when exports are handled on the exporting library.
-  var reexportingLibrary = compiler.libraryLoader.loadLibrary(
-      reexportingLibraryUri, null, reexportingLibraryUri);
-  foo = findInExports(reexportingLibrary, 'foo');
-  Expect.isNotNull(foo);
-  Expect.isTrue(foo.isField());
+  compiler.libraryLoader.loadLibrary(
+      exportingLibraryUri, null, exportingLibraryUri).then((exportingLibrary) {
+    Expect.isTrue(exportingLibrary.exportsHandled);
+    var foo = findInExports(exportingLibrary, 'foo');
+    Expect.isNotNull(foo);
+    Expect.isTrue(foo.isField());
+
+    // Load reexporting library when exports are handled on the exporting library.
+    return compiler.libraryLoader.loadLibrary(
+        reexportingLibraryUri, null, reexportingLibraryUri);
+  }).then((reexportingLibrary) {
+    var foo = findInExports(reexportingLibrary, 'foo');
+    Expect.isNotNull(foo);
+    Expect.isTrue(foo.isField());
+  });
 }
 
 Element findInExports(LibraryElement library, String name) {