First pass at asynchronous input loading in dart2js.

tests/compiler/dart2js/memory_source_file_helper.dart

Index: tests/compiler/dart2js/memory_source_file_helper.dart
diff --git a/tests/compiler/dart2js/memory_source_file_helper.dart b/tests/compiler/dart2js/memory_source_file_helper.dart
index 987fff71930c57603ef5205942e63a51016583b3..7f27bbf0ee10eb1e19f4be5f2934fe6b22fbd0c9 100644
--- a/tests/compiler/dart2js/memory_source_file_helper.dart
+++ b/tests/compiler/dart2js/memory_source_file_helper.dart
@@ -31,7 +31,7 @@ class MemorySourceFileProvider extends SourceFileProvider {
     }
     String source = MEMORY_SOURCE_FILES[resourceUri.path];
     // TODO(ahe): Return new Future.error(...) ?
-    if (source == null) throw 'No such file $resourceUri';
+    if (source == null) return new Future.error('No such file $resourceUri');
     String resourceName = '$resourceUri';
     this.sourceFiles[resourceName] = new SourceFile(resourceName, source);
     return new Future.value(source);