Only run [declareOutputs] once for each asset/transformer pair.

pkg/barback/lib/src/base_transform.dart

Index: pkg/barback/lib/src/base_transform.dart
diff --git a/pkg/barback/lib/src/base_transform.dart b/pkg/barback/lib/src/base_transform.dart
index c0de0f8ac26d5ee171f273cf6e593087aeaa6e3f..bb0183c5d74be119b6a0358a6fc70774dda9b3b8 100644
--- a/pkg/barback/lib/src/base_transform.dart
+++ b/pkg/barback/lib/src/base_transform.dart
@@ -5,16 +5,10 @@
 library barback.base_transform;
 
 import 'dart:async';
-import 'dart:convert';
 
-import 'asset.dart';
-import 'asset_id.dart';
-import 'asset_node.dart';
-import 'errors.dart';
 import 'log.dart';
 import 'transform_logger.dart';
 import 'transform_node.dart';
-import 'utils.dart';
 
 /// The base class for the ephemeral transform objects that are passed to
 /// transformers.
@@ -47,29 +41,6 @@ abstract class BaseTransform {
   TransformLogger get logger => _logger;
   TransformLogger _logger;
 
-  /// Gets the primary input asset.
-  ///
-  /// While a transformation can use multiple input assets, one must be a
-  /// special "primary" asset. This will be the "entrypoint" or "main" input
-  /// file for a transformation.
-  ///
-  /// For example, with a dart2js transform, the primary input would be the
-  /// entrypoint Dart file. All of the other Dart files that that imports
-  /// would be secondary inputs.
-  ///
-  /// This method may fail at runtime with an [AssetNotFoundException] if called
-  /// asynchronously after the transform begins running. The primary input may
-  /// become unavailable while this transformer is running due to asset changes
-  /// earlier in the graph. You can ignore the error if this happens: the
-  /// transformer will be re-run automatically for you.
-  Asset get primaryInput {
-    if (_node.primary.state != AssetState.AVAILABLE) {
-      throw new AssetNotFoundException(_node.primary.id);
-    }
-
-    return _node.primary.asset;
-  }
-
   BaseTransform(this._node) {
     _logger = new TransformLogger((asset, level, message, span) {
       if (level == LogLevel.ERROR) _loggedError = true;
@@ -81,46 +52,6 @@ abstract class BaseTransform {
     });
   }
 
-  /// Gets the asset for an input [id].
-  ///
-  /// If an input with [id] cannot be found, throws an [AssetNotFoundException].
-  Future<Asset> getInput(AssetId id) => _node.getInput(id);
-
-  /// A convenience method to the contents of the input with [id] as a string.
-  ///
-  /// This is equivalent to calling [getInput] followed by [Asset.readAsString].
-  ///
-  /// If the asset was created from a [String] the original string is always
-  /// returned and [encoding] is ignored. Otherwise, the binary data of the
-  /// asset is decoded using [encoding], which defaults to [UTF8].
-  ///
-  /// If an input with [id] cannot be found, throws an [AssetNotFoundException].
-  Future<String> readInputAsString(AssetId id, {Encoding encoding}) {
-    if (encoding == null) encoding = UTF8;
-    return getInput(id).then((input) => input.readAsString(encoding: encoding));
-  }
-
-  /// A convenience method to the contents of the input with [id].
-  ///
-  /// This is equivalent to calling [getInput] followed by [Asset.read].
-  ///
-  /// If the asset was created from a [String], this returns its UTF-8 encoding.
-  ///
-  /// If an input with [id] cannot be found, throws an [AssetNotFoundException].
-  Stream<List<int>> readInput(AssetId id) =>
-      futureStream(getInput(id).then((input) => input.read()));
-
-  /// A convenience method to return whether or not an asset exists.
-  ///
-  /// This is equivalent to calling [getInput] and catching an
-  /// [AssetNotFoundException].
-  Future<bool> hasInput(AssetId id) {
-    return getInput(id).then((_) => true).catchError((error) {
-      if (error is AssetNotFoundException && error.id == id) return false;
-      throw error;
-    });
-  }
-
   /// Consume the primary input so that it doesn't get processed by future
   /// phases or emitted once processing has finished.
   ///