Improve pub's pubspec error messages.

sdk/lib/_internal/pub/bin/pub.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.
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:args/args.dart';
 import 'package:http/http.dart' as http;
 import 'package:path/path.dart' as path;
+import 'package:source_maps/source_maps.dart';
 import 'package:stack_trace/stack_trace.dart';
 
 import '../lib/src/command.dart';
 import '../lib/src/exit_codes.dart' as exit_codes;
 import '../lib/src/http.dart';
 import '../lib/src/io.dart';
 import '../lib/src/log.dart' as log;
 import '../lib/src/sdk.dart' as sdk;
 import '../lib/src/utils.dart';
 
@@ skipping 58 matching lines @@
 void runPub(String cacheDir, ArgResults options, List<String> arguments) {
   var captureStackChains =
       options['trace'] ||
       options['verbose'] ||
       options['verbosity'] == 'all';
 
   captureErrors(() => invokeCommand(cacheDir, options),
       captureStackChains: captureStackChains).catchError((error, Chain chain) {
     // This is basically the top-level exception handler so that we don't
     // spew a stack trace on our users.
-    var message = getErrorMessage(error);
-    log.error(message);
+    if (error is SpanException) {
+      log.error(error.toString(useColors: canUseSpecialChars));
+    } else {
+      log.error(getErrorMessage(error));
+    }
     log.fine("Exception type: ${error.runtimeType}");
 
     if (log.json.enabled) {
       if (error is UsageException) {
         // Don't print usage info in JSON output.
         log.json.error(error.message);
       } else {
         log.json.error(error);
       }
     }
@@ skipping 107 matching lines @@
     if (Platform.operatingSystem != 'windows') return null;
 
     return runProcess('ver', []).then((result) {
       if (result.stdout.join('\n').contains('XP')) {
         log.error('Sorry, but pub is not supported on Windows XP.');
         return flushThenExit(exit_codes.USAGE);
       }
     });
   });
 }