Step one towards stable error messages with details:

pkg/polymer/lib/src/build/wrapped_logger.dart

-// Copyright (c) 2013, 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.
-
-library polymer.src.build.wrapped_logger;
-
-import 'dart:async';
-import 'dart:convert';
-
-import 'package:barback/barback.dart';
-import 'package:source_span/source_span.dart';
-
-import 'common.dart' as common;
-
-/// A simple class to wrap one TransformLogger with another one that writes all
-/// logs to a file and then forwards the calls to the child.
-class WrappedLogger implements TransformLogger {
-  Transform _transform;
-  List<Map> _logs = new List<Map>();
-
-  bool convertErrorsToWarnings;
-
-  WrappedLogger(this._transform, {this.convertErrorsToWarnings: false});
-
-  void info(String message, {AssetId asset, SourceSpan span}) {
-    _transform.logger.info(message, asset: asset, span: span);
-    _addLog(asset, LogLevel.INFO, message, span);
-  }
-
-  void fine(String message, {AssetId asset, SourceSpan span}) {
-    _transform.logger.fine(message, asset: asset, span: span);
-    _addLog(asset, LogLevel.FINE, message, span);
-  }
-
-  void warning(String message, {AssetId asset, SourceSpan span}) {
-    _transform.logger.warning(message, asset: asset, span: span);
-    _addLog(asset, LogLevel.WARNING, message, span);
-  }
-
-  void error(String message, {AssetId asset, SourceSpan span}) {
-    if (convertErrorsToWarnings) {
-      _transform.logger.warning(message, asset: asset, span: span);
-    } else {
-      _transform.logger.error(message, asset: asset, span: span);
-    }
-    _addLog(asset, LogLevel.ERROR, message, span);
-  }
-
-  /// Outputs the log data to a JSON serialized file.
-  Future writeOutput() {
-    return getNextLogAssetPath().then((path) {
-      _transform.addOutput(new Asset.fromString(path, JSON.encode(_logs)));
-    });
-  }
-
-  // Each phase outputs a new log file with an incrementing # appended, this
-  // figures out the next # to use.
-  Future<String> getNextLogAssetPath([int nextNumber = 1]) {
-    var nextAssetPath = _transform.primaryInput.id.addExtension(
-        '${common.LOG_EXTENSION}.$nextNumber');
-    return _transform.hasInput(nextAssetPath).then((exists) {
-      if (!exists) return nextAssetPath;
-      return getNextLogAssetPath(++nextNumber);
-    });
-  }
-
-  // Reads all log files for an Asset into [logs].
-  static Future _readLogFilesForAsset(
-      AssetId id, Transform transform, List<Map> logs, [nextNumber = 1]) {
-    var nextAssetPath = id.addExtension('${common.LOG_EXTENSION}.$nextNumber');
-    return transform.hasInput(nextAssetPath).then((exists) {
-      if (!exists) return null;
-      return transform.readInputAsString(nextAssetPath).then((data) {
-        logs.addAll(JSON.decode(data));
-        return _readLogFilesForAsset(id, transform, logs, ++nextNumber);
-      });
-    });
-  }
-
-  // Combines all existing ._buildLogs.* files into a single ._buildLogs file.
-  static Future combineLogFiles(Transform transform) {
-    var logs = new List<Map>();
-    var id = transform.primaryInput.id;
-    return _readLogFilesForAsset(id, transform, logs).then((_) {
-      return transform.addOutput(new Asset.fromString(
-          id.addExtension(common.LOG_EXTENSION),
-          JSON.encode(logs)));
-    });
-  }
-
-  // Reads all logs for an asset and adds them to this loggers log output.
-  Future addLogFilesFromAsset(AssetId id, [int nextNumber = 1]) {
-    return _readLogFilesForAsset(id, _transform, _logs);
-  }
-
-  void _addLog(AssetId assetId, LogLevel level, String message,
-               SourceSpan span) {
-    var data = {
-        'level': level.name,
-        'message': const HtmlEscape().convert(message),
-    };
-    if (assetId != null) {
-      data['assetId'] = {
-          'package': assetId.package,
-          'path': assetId.path,
-      };
-    }
-    if (span != null) {
-      data['span'] = {
-          'location': span.start.toolString,
-          'text': new HtmlEscape().convert(span.text),
-      };
-    }
-    _logs.add(data);
-  }
-}