Explicitly configure git committer.

utils/pub/io.dart

Index: utils/pub/io.dart
diff --git a/utils/pub/io.dart b/utils/pub/io.dart
index c2f14568906fd92033c75fdd9e71d1b2c4990d43..3eda416760fab8ff9adfd3a98ca562464b9aacec 100644
--- a/utils/pub/io.dart
+++ b/utils/pub/io.dart
@@ -499,7 +499,10 @@ Future<PubProcessResult> runProcess(String executable, List<String> args,
   if (workingDir != null) {
     options.workingDirectory = _getDirectory(workingDir).path;
   }
-  options.environment = environment;
+
+  if (environment != null) {
+    options.environment = environment;
+  }
 
   var future = Process.run(executable, args, options);
   return future.transform((result) {
@@ -556,8 +559,11 @@ Future<bool> get isGitInstalled {
 }
 
 /// Run a git process with [args] from [workingDir].
-Future<PubProcessResult> runGit(List<String> args, {String workingDir}) =>
-    _gitCommand.chain((git) => runProcess(git, args, workingDir: workingDir));
+Future<PubProcessResult> runGit(List<String> args,
+    {String workingDir, Map<String, String> environment}) {
+  return _gitCommand.chain((git) => runProcess(git, args,
+        workingDir: workingDir, environment: environment));
+}
 
 /// Returns the name of the git command-line app, or null if Git could not be
 /// found on the user's PATH.