First pass at asynchronous input loading in dart2js.

tests/compiler/dart2js/async_compiler_input_provider_test.dart

+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import "dart:async";
+import "dart:io";
+
+import "package:expect/expect.dart";
+
+import '../../../sdk/lib/_internal/compiler/compiler.dart' as compiler;
+import '../../../sdk/lib/_internal/compiler/implementation/filenames.dart';
+
+const SOURCES = const {
+  "/main.dart": """
+    import "foo.dart";
+    main() => foo();
+    """,
+  "/foo.dart": """
+    library foo;
+    import "bar.dart";
+    foo() => bar();
+    """,
+  "/bar.dart": """
+    library bar;
+    bar() => print("bar");
+    """
+};
+
+Future<String> provideInput(Uri uri) {
+  var source = SOURCES[uri.path];
+  if (source == null) {
+    // Not one of our source files, so assume it's a built-in.
+    source = new File(uriPathToNative(uri.path)).readAsStringSync();
+  }
+
+  // Deliver the input asynchronously.
+  return new Future(() => source);
+}
+
+main() {
+  var entrypoint = Uri.parse("file:///main.dart");
+
+  // Find the path to sdk/ in the repo relative to this script.
+  Uri script = currentDirectory.resolve(nativeToUriPath(Platform.script));
+  Uri libraryRoot = script.resolve('../../../sdk/');
+  Uri packageRoot = script.resolve('./packages/');
+
+  compiler.compile(entrypoint, libraryRoot, packageRoot,
+      provideInput, handleDiagnostic, []).then((code) {
+    Expect.isNotNull(code);
+  });
+}
+
+void handleDiagnostic(Uri uri, int begin, int end, String message,
+                      compiler.Diagnostic kind) {
+  print(message);
+  // Do nothing.
+}