First pass at asynchronous input loading in dart2js.

sdk/lib/_internal/compiler/implementation/mirrors/dart2js_mirror.dart

Index: sdk/lib/_internal/compiler/implementation/mirrors/dart2js_mirror.dart
diff --git a/sdk/lib/_internal/compiler/implementation/mirrors/dart2js_mirror.dart b/sdk/lib/_internal/compiler/implementation/mirrors/dart2js_mirror.dart
index 2910563d6cb68ec18d23bc0ca17ef9c9dd11a3aa..2e77d87ae537acfdf4a3e1d974e9c0278cb4a1e7 100644
--- a/sdk/lib/_internal/compiler/implementation/mirrors/dart2js_mirror.dart
+++ b/sdk/lib/_internal/compiler/implementation/mirrors/dart2js_mirror.dart
@@ -226,12 +226,13 @@ Future<MirrorSystem> analyze(List<Uri> libraries,
                                            internalDiagnosticHandler,
                                            libraryRoot, packageRoot, options);
   compiler.librariesToAnalyzeWhenRun = libraries;
-  bool success = compiler.run(null);
-  if (success && !compilationFailed) {
-    return new Future<MirrorSystem>.value(new Dart2JsMirrorSystem(compiler));
-  } else {
-    return new Future<MirrorSystem>.error('Failed to create mirror system.');
-  }
+  return compiler.run(null).then((bool success) {
+    if (success && !compilationFailed) {
+      return new Dart2JsMirrorSystem(compiler);
+    } else {
+      throw 'Failed to create mirror system.';
+    }
+  });
 }
 
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