Step one towards stable error messages with details:

pkg/code_transformers/lib/messages/build_logger.dart

Index: pkg/code_transformers/lib/messages/build_logger.dart
diff --git a/pkg/code_transformers/lib/messages/build_logger.dart b/pkg/code_transformers/lib/messages/build_logger.dart
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+// 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 code_transformers.messages.messages_logger;
+
+import 'dart:async';
+import 'dart:convert' show JSON;
+
+import 'package:barback/barback.dart';
+import 'package:source_span/source_span.dart';
+
+import 'messages.dart' show Message, MessageId, BuildLogEntry, LogEntryTable;
+
+/// A [TransformLogger] used to track error and warning messages produced during
+/// a build.
+///
+/// This logger records all messages that were logged and then forwards
+/// the calls to an underlying [TransformLogger]. The internal records support
+/// serializing the errors and emiting them to an asset (so they can be
+/// presented to the user in a web-based client), clustering similar messages
+/// together, sorting messages in order of importance, etc.
+///
+/// The logger also supports reporting error messages as warnings. Barback makes
+/// error messages stop the transformation process, which sometimes can surprise
+/// users. Turning errors into warnings is especially useful when used within
+/// `pub serve`, where we would like the transformation to continue as far as it
+/// can. When this flag is turned on, the level is still recorded as an error,
+/// so a web client UI can still highlight their importance.
+// TODO(sigmund): also cluster messages when they are reported on the
+// command-line.
+class BuildLogger implements TransformLogger {
+  /// Underling transform that is currently active.
+  final Transform _transform;
+
+  /// Logs created during the current transform.
+  final LogEntryTable _logs = new LogEntryTable();
+
+  /// Whether to use `warning` or `error` when forwarding error messages to the
+  /// underlying logger in `_transform.logger`.
+  final bool convertErrorsToWarnings;
+
+  BuildLogger(this._transform, {this.convertErrorsToWarnings: false});
+
+  /// Records a message at the fine level. If [msg] is a [Message] it is
+  /// recorded directly, otherwise it is first converted to a [String].
+  void fine(msg, {AssetId asset, SourceSpan span}) {
+    msg = msg is Message ? msg : new Message.unknown('$msg');
+    _transform.logger.fine(msg.snippet, asset: asset, span: span);
+    _logs.add(new BuildLogEntry(msg, span, LogLevel.FINE.name));
+  }
+
+  /// Records a message at the info level. If [msg] is a [Message] it is
+  /// recorded directly, otherwise it is first converted to a [String].
+  void info(msg, {AssetId asset, SourceSpan span}) {
+    msg = msg is Message ? msg : new Message.unknown('$msg');
+    _transform.logger.info(msg.snippet, asset: asset, span: span);
+    _logs.add(new BuildLogEntry(msg, span, LogLevel.INFO.name));
+  }
+
+  /// Records a warning message. If [msg] is a [Message] it is recorded
+  /// directly, otherwise it is first converted to a [String].
+  void warning(msg, {AssetId asset, SourceSpan span}) {
+    msg = msg is Message ? msg : new Message.unknown('$msg');
+    _transform.logger.warning(msg.snippet, asset: asset, span: span);
+    _logs.add(new BuildLogEntry(msg, span, LogLevel.WARNING.name));
+  }
+
+  /// Records an error message. If [msg] is a [Message] it is recorded
+  /// directly, otherwise it is first converted to a [String].
+  void error(msg, {AssetId asset, SourceSpan span}) {
+    msg = msg is Message ? msg : new Message.unknown('$msg');
+    if (convertErrorsToWarnings) {
+      _transform.logger.warning(msg.snippet, asset: asset, span: span);
+    } else {
+      _transform.logger.error(msg.snippet, asset: asset, span: span);
+    }
+    _logs.add(new BuildLogEntry(msg, span, LogLevel.ERROR.name));
+  }
+
+  /// 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(
+        '${LOG_EXTENSION}.$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,
+      LogEntryTable entries, [nextNumber = 1]) {
+    var nextAssetPath = id.addExtension('${LOG_EXTENSION}.$nextNumber');
+    return transform.hasInput(nextAssetPath).then((exists) {
+      if (!exists) return null;
+      return transform.readInputAsString(nextAssetPath).then((data) {
+        entries.addAll(new LogEntryTable.fromJson(JSON.decode(data)));
+        return _readLogFilesForAsset(id, transform, entries, ++nextNumber);
+      });
+    });
+  }
+
+  // Combines all existing ._buildLogs.* files into a single ._buildLogs file.
+  static Future combineLogFiles(Transform transform) {
+    var entries = new LogEntryTable();
+    var id = transform.primaryInput.id;
+    return _readLogFilesForAsset(id, transform, entries).then((_) {
+      return transform.addOutput(new Asset.fromString(
+          id.addExtension(LOG_EXTENSION),
+          JSON.encode(entries.toJson())));
+    });
+  }
+
+  // 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);
+  }
+}
+
+/// Extension used for assets that contained serialized logs.
+const String LOG_EXTENSION = '._buildLogs';