Revised attempt at browser logging

lib/src/async_import_cache.dart

@@ -84,12 +84,12 @@ class AsyncImportCache {
       PackageConfig? packageConfig,
       Logger? logger})
       : _importers = _toImporters(importers, loadPaths, packageConfig),
-        _logger = logger ?? const Logger.stderr();
+        _logger = logger ?? Logger.stderr();
 
   /// Creates an import cache without any globally-available importers.
   AsyncImportCache.none({Logger? logger})
       : _importers = const [],
-        _logger = logger ?? const Logger.stderr();
+        _logger = logger ?? Logger.stderr();
 
   /// Converts the user's [importers], [loadPaths], and [packageConfig]
   /// options into a single list of importers.

lib/src/import_cache.dart

@@ -5,7 +5,7 @@
 // DO NOT EDIT. This file was generated from async_import_cache.dart.
 // See tool/grind/synchronize.dart for details.
 //
-// Checksum: 1273c347383da4775032276417fd7568c7e5330b
+// Checksum: 62a3680f744d9ee34989dac0980f788ecf694070
 //
 // ignore_for_file: unused_import
 
@@ -90,12 +90,12 @@ class ImportCache {
       PackageConfig? packageConfig,
       Logger? logger})
       : _importers = _toImporters(importers, loadPaths, packageConfig),
-        _logger = logger ?? const Logger.stderr();
+        _logger = logger ?? Logger.stderr();
 
   /// Creates an import cache without any globally-available importers.
   ImportCache.none({Logger? logger})
       : _importers = const [],
-        _logger = logger ?? const Logger.stderr();
+        _logger = logger ?? Logger.stderr();
 
   /// Converts the user's [importers], [loadPaths], and [packageConfig]
   /// options into a single list of importers.

lib/src/io/interface.dart

@@ -25,6 +25,15 @@ class FileSystemException {
   String? get path => throw '';
 }
 
+abstract class Console {
+  external void log(String data);
+  external void info(String data);
+  external void warn(String data);
+  external void error(String data);
+}
+
+Console? get console => throw '';
+
 /// The standard error for the current process.
 Stderr get stderr => throw '';
 

lib/src/io/node.dart

@@ -7,8 +7,9 @@ import 'dart:convert';
 import 'dart:js_util';
 
 import 'package:js/js.dart';
+import 'package:node_interop/console.dart';
 import 'package:node_interop/fs.dart';
-import 'package:node_interop/node_interop.dart' hide process;
+import 'package:node_interop/node_interop.dart' as n hide process;
 import 'package:node_interop/stream.dart';
 import 'package:path/path.dart' as p;
 import 'package:source_span/source_span.dart';
@@ -18,7 +19,9 @@ import '../exception.dart';
 import '../node/chokidar.dart';
 
 @JS('process')
-external final Process? process; // process is null in the browser
+external final n.Process? process; // process is null in the browser
+
+Console? get console => n.console;
 
 class FileSystemException {
   final String message;
@@ -30,24 +33,14 @@ class FileSystemException {
 }
 
 class Stderr {
-  final Writable? _stderr;
+  final Writable _stderr;
 
   Stderr(this._stderr);
 
-  void write(Object object) {
-    if (_stderr == null) {
-      console.error(object.toString());
-    } else {
-      _stderr?.write(object.toString());
-    }
-  }
+  void write(Object object) => _stderr.write(object.toString());
 
   void writeln([Object? object]) {
-    if (_stderr == null) {
-      console.error("${object ?? ''}\n");
-    } else {
-      _stderr?.write("${object ?? ''}\n");
-    }
+    _stderr.write("${object ?? ''}\n");
   }
 
   void flush() {}
@@ -122,7 +115,7 @@ Future<String> readStdin() async {
 }
 
 /// Cleans up a Node system error's message.
-String _cleanErrorMessage(JsSystemError error) {
+String _cleanErrorMessage(n.JsSystemError error) {
   // The error message is of the form "$code: $text, $syscall '$path'". We just
   // want the text.
   return error.message.substring("${error.code}: ".length,
@@ -143,7 +136,7 @@ bool fileExists(String path) {
     try {
       return fs.statSync(path).isFile();
     } catch (error) {
-      var systemError = error as JsSystemError;
+      var systemError = error as n.JsSystemError;
       if (systemError.code == 'ENOENT') return false;
       rethrow;
     }
@@ -164,7 +157,7 @@ bool dirExists(String path) {
     try {
       return fs.statSync(path).isDirectory();
     } catch (error) {
-      var systemError = error as JsSystemError;
+      var systemError = error as n.JsSystemError;
       if (systemError.code == 'ENOENT') return false;
       rethrow;
     }
@@ -179,7 +172,7 @@ void ensureDir(String path) {
     try {
       fs.mkdirSync(path);
     } catch (error) {
-      var systemError = error as JsSystemError;
+      var systemError = error as n.JsSystemError;
       if (systemError.code == 'EEXIST') return;
       if (systemError.code != 'ENOENT') rethrow;
       ensureDir(p.dirname(path));
@@ -229,15 +222,15 @@ T _systemErrorToFileSystemException<T>(T callback()) {
   try {
     return callback();
   } catch (error) {
-    if (error is! JsSystemError) rethrow;
+    if (error is! n.JsSystemError) rethrow;
     throw FileSystemException._(_cleanErrorMessage(error), error.path);
   }
 }
 
 final Stderr stderr = () {
   var stderr_ = process?.stderr;
   if (stderr_ == null) {
-    return Stderr(null);
+    throw UnsupportedError("stderr is only supported on Node.js");
   }
   return Stderr(stderr_);
 }();

lib/src/io/vm.dart

@@ -18,6 +18,15 @@ export 'dart:io' show exitCode, FileSystemException;
 
 io.Stdout get stderr => io.stderr;
 
+abstract class Console {
+  external void log(String data);
+  external void info(String data);
+  external void warn(String data);
+  external void error(String data);
+}
+
+Console? get console => throw '';
+
 bool get isWindows => io.Platform.isWindows;
 
 bool get isMacOS => io.Platform.isMacOS;

lib/src/logger.dart

@@ -7,7 +7,9 @@ import 'package:source_span/source_span.dart';
 import 'package:stack_trace/stack_trace.dart';
 
 import 'deprecation.dart';
+import 'io.dart';
 import 'logger/deprecation_handling.dart';
+import 'logger/browser.dart';
 import 'logger/stderr.dart';
 
 /// An interface for loggers that print messages produced by Sass stylesheets.
@@ -21,7 +23,12 @@ abstract class Logger {
 
   /// Creates a logger that prints warnings to standard error, with terminal
   /// colors if [color] is `true` (default `false`).
-  const factory Logger.stderr({bool color}) = StderrLogger;
+  factory Logger.stderr({bool color = false}) {
+    if (isBrowser) {
+      return BrowserLogger(color: color);
+    }
+    return StderrLogger(color: color);
+  }
 
   /// Emits a warning with the given [message].
   ///

lib/src/logger/browser.dart

@@ -0,0 +1,56 @@
+// Copyright 2018 Google Inc. Use of this source code is governed by an
+// MIT-style license that can be found in the LICENSE file or at
+// https://opensource.org/licenses/MIT.
+
+import 'package:path/path.dart' as p;
+import 'package:source_span/source_span.dart';
+import 'package:stack_trace/stack_trace.dart';
+
+import '../io.dart';
+import '../logger.dart';
+import '../utils.dart';
+
+/// A logger that prints warnings to browser console.
+class BrowserLogger implements Logger {
+  /// Whether to use colors in messages.
+  final bool color;
+
+  const BrowserLogger({this.color = false});
+
+  void warn(String message,
+      {FileSpan? span, Trace? trace, bool deprecation = false}) {
+    var result = '';
+    if (color) {
+      // Bold yellow.
+      result += '\u001b[33m\u001b[1m';
+      if (deprecation) result += 'Deprecation ';
+      result += 'Warning\u001b[0m';
+    } else {
+      if (deprecation) result += 'DEPRECATION ';
+      result += 'WARNING';
+    }
+
+    if (span == null) {
+      result += ': $message\n';
+    } else if (trace != null) {
+      // If there's a span and a trace, the span's location information is
+      // probably duplicated in the trace, so we just use it for highlighting.
+      result += ': $message\n\n${span.highlight(color: color)}\n';
+    } else {
+      result += ' on ${span.message("\n" + message, color: color)}\n';
+    }
+
+    if (trace != null) result += '${indent(trace.toString().trimRight(), 4)}\n';
+    console?.error(result);
+  }
+
+  void debug(String message, SourceSpan span) {
+    var result = '';
+    var url =
+        span.start.sourceUrl == null ? '-' : p.prettyUri(span.start.sourceUrl);
+    result += '$url:${span.start.line + 1} ';
+    result += color ? '\u001b[1mDebug\u001b[0m' : 'DEBUG';
+    result += ': $message\n';
+    console?.warn(result);
+  }
+}

lib/src/parse/parser.dart

@@ -58,7 +58,7 @@ class Parser {
   Parser(String contents,
       {Object? url, Logger? logger, InterpolationMap? interpolationMap})
       : scanner = SpanScanner(contents, sourceUrl: url),
-        logger = logger ?? const Logger.stderr(),
+        logger = logger ?? Logger.stderr(),
         _interpolationMap = interpolationMap;
 
   String _parseIdentifier() {

lib/src/visitor/async_evaluate.dart

@@ -332,7 +332,7 @@ class _EvaluateVisitor
             ? importCache ?? AsyncImportCache.none(logger: logger)
             : null,
         _nodeImporter = nodeImporter,
-        _logger = logger ?? const Logger.stderr(),
+        _logger = logger ?? Logger.stderr(),
         _quietDeps = quietDeps,
         _sourceMap = sourceMap,
         // The default environment is overridden in [_execute] for full

lib/src/visitor/evaluate.dart

@@ -5,7 +5,7 @@
 // DO NOT EDIT. This file was generated from async_evaluate.dart.
 // See tool/grind/synchronize.dart for details.
 //
-// Checksum: 3e5e09dec7a8bcc6bc017103c67f463843b7fed7
+// Checksum: ebae617f3ffce6fd036874e06626c391d37a962b
 //
 // ignore_for_file: unused_import
 
@@ -340,7 +340,7 @@ class _EvaluateVisitor
             ? importCache ?? ImportCache.none(logger: logger)
             : null,
         _nodeImporter = nodeImporter,
-        _logger = logger ?? const Logger.stderr(),
+        _logger = logger ?? Logger.stderr(),
         _quietDeps = quietDeps,
         _sourceMap = sourceMap,
         // The default environment is overridden in [_execute] for full

test/dart_api/logger_test.dart

@@ -240,9 +240,9 @@ class _TestLogger implements Logger {
       _warn;
   final void Function(String, SourceSpan) _debug;
 
-  _TestLogger.withWarn(this._warn) : _debug = const Logger.stderr().debug;
+  _TestLogger.withWarn(this._warn) : _debug = Logger.stderr().debug;
 
-  _TestLogger.withDebug(this._debug) : _warn = const Logger.stderr().warn;
+  _TestLogger.withDebug(this._debug) : _warn = Logger.stderr().warn;
 
   void warn(String message,
           {FileSpan? span, Trace? trace, bool deprecation = false}) =>