Polymer transformer logs now show on the frontend for pub serve.

pkg/polymer/lib/src/build/common.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.
 
 /// Common methods used by transfomers.
 library polymer.src.build.common;
 
 import 'dart:async';
 
 import 'package:analyzer/src/generated/ast.dart';
@@ skipping 59 matching lines @@
   /// `type="application/dart"` scripts with equivalent *.dart.js files.
   ///
   /// If [contentSecurityPolicy] enabled, this will reference files
   /// named *.dart.precompiled.js.
   final bool directlyIncludeJS;
 
   /// Run transformers to create a releasable app. For example, include the
   /// minified versions of the polyfills rather than the debug versions.
   final bool releaseMode;
 
+  /// This will make a physical element appear on the page showing build logs.
+  /// It will only appear when ![releaseMode] even if this is true.
+  final bool injectBuildLogsInOutput;
+
   /// True to run liner on all html files before starting other phases.
   // TODO(jmesserly): instead of this flag, we should only run linter on
   // reachable (entry point+imported) html if deploying. See dartbug.com/17199.
   final bool lint;
 
   TransformOptions({entryPoints, this.inlineStylesheets,
       this.contentSecurityPolicy: false, this.directlyIncludeJS: true,
-      this.releaseMode: true, this.lint: true})
+      this.releaseMode: true, this.lint: true,
+      this.injectBuildLogsInOutput: false})
       : entryPoints = entryPoints == null ? null
           : entryPoints.map(systemToAssetPath).toList();
 
   /// Whether an asset with [id] is an entry point HTML file.
   bool isHtmlEntryPoint(AssetId id) {
     if (id.extension != '.html') return false;
 
     // Note: [id.path] is a relative path from the root of a package.
     if (entryPoints == null) {
       return id.path.startsWith('web/') || id.path.startsWith('test/');
@@ skipping 125 matching lines @@
 bool isCustomTagName(String name) {
   if (name == null || !name.contains('-')) return false;
   return !invalidTagNames.containsKey(name);
 }
 
 /// Regex to split names in the 'attributes' attribute, which supports 'a b c',
 /// 'a,b,c', or even 'a b,c'. This is the same as in `lib/src/declaration.dart`.
 final ATTRIBUTES_REGEX = new RegExp(r'\s|,');
 
 const POLYMER_EXPERIMENTAL_HTML = 'packages/polymer/polymer_experimental.html';
+
+const String LOG_EXTENSION = '._buildLogs';