First pass at asynchronous input loading in dart2js.

tests/compiler/dart2js/tree_shaking_test.dart

Index: tests/compiler/dart2js/tree_shaking_test.dart
diff --git a/tests/compiler/dart2js/tree_shaking_test.dart b/tests/compiler/dart2js/tree_shaking_test.dart
index 51580156a3a598901f66911c64d8ab8fef84124b..a7f5294236114181af45020a849799854fa50668 100644
--- a/tests/compiler/dart2js/tree_shaking_test.dart
+++ b/tests/compiler/dart2js/tree_shaking_test.dart
@@ -25,8 +25,9 @@ main() {
 """;
 
 void main() {
-  String generated = compileAll(TEST);
-  Expect.isTrue(generated.contains('return 42'));
-  Expect.isTrue(generated.contains('return 54'));
-  Expect.isFalse(generated.contains('return 68'));
+  compileAll(TEST).then((generated) {
+    Expect.isTrue(generated.contains('return 42'));
+    Expect.isTrue(generated.contains('return 54'));
+    Expect.isFalse(generated.contains('return 68'));
+  });
 }