Fix a Debian build error.

lib/src/command_runner.dart

 // Copyright (c) 2014, 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 pub.command_runner;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:args/args.dart';
@@ skipping 142 matching lines @@
   void printUsage() {
     log.message(usage);
   }
 
   /// Print a warning if we're running from the Dart SDK repo and pub isn't
   /// up-to-date.
   ///
   /// This is otherwise hard to tell, and can produce confusing behavior issues.
   void _checkDepsSynced() {
     if (!runningFromDartRepo) return;
+    if (!git.isInstalled) return;
 
     var deps = readTextFile(p.join(dartRepoRoot, 'DEPS'));
     var pubRevRegExp = new RegExp(
         r'^ +"pub_rev": +"@([^"]+)"', multiLine: true);
     var match = pubRevRegExp.firstMatch(deps);
     if (match == null) return;
     var depsRev = match[1];
 
-    var actualRev = git.runSync(["rev-parse", "HEAD"], workingDir: pubRoot)
-        .single;
+    var actualRev;
+    try {
+      actualRev = git.runSync(["rev-parse", "HEAD"], workingDir: pubRoot)
+          .single;
+    } on git.GitException catch (_) {
+      // When building for Debian, pub isn't checked out via git.
+      return;
+    }
 
     if (depsRev == actualRev) return;
     log.warning(
         "${log.yellow('Warning:')} the revision of pub in DEPS is "
             "${log.bold(depsRev)},\n"
         "but ${log.bold(actualRev)} is checked out in "
             "${p.relative(pubRoot)}.\n\n");
   }
 
   /// Returns the appropriate exit code for [exception], falling back on 1 if no
@@ skipping 21 matching lines @@
   Future _validatePlatform() async {
     if (Platform.operatingSystem != 'windows') return;
 
     var result = await runProcess('ver', []);
     if (result.stdout.join('\n').contains('XP')) {
       log.error('Sorry, but pub is not supported on Windows XP.');
       await flushThenExit(exit_codes.USAGE);
     }
   }
 }