feat(dart/transform): Allow multiple transformer entry points

- Allow the user to specify multiple entry points to an app.
- Allow the Angular 2 transformer to run without explicit entry points to
generate necessary setters & getters on built-in directives like `For`
and `If`.

Closes #1246

modules/angular2/src/transform/common/options_reader.dart

@@ -0,0 +1,36 @@
+library angular2.transform.common.options;
+
+import 'package:barback/barback.dart';
+import 'options.dart';
+
+TransformerOptions parseBarbackSettings(BarbackSettings settings) {
+  var config = settings.configuration;
+  var entryPoints = _readFileList(config, ENTRY_POINT_PARAM);
+  var reflectionEntryPoints =
+      _readFileList(config, REFLECTION_ENTRY_POINT_PARAM);
+  return new TransformerOptions(entryPoints,
+      reflectionEntryPoints: reflectionEntryPoints,
+      modeName: settings.mode.name);
+}
+
+/// Cribbed from the polymer project.
+/// [https://github.com/dart-lang/polymer-dart]
+List<String> _readFileList(Map config, String paramName) {
+  var value = config[paramName];
+  if (value == null) return null;
+  var files = [];
+  bool error = false;
+  if (value is List) {
+    files = value;
+    error = value.any((e) => e is! String);
+  } else if (value is String) {
+    files = [value];
+    error = false;
+  } else {
+    error = true;
+  }
+  if (error) {
+    print('Invalid value for "$paramName" in the Angular 2 transformer.');
+  }
+  return files;
+}