Ensure that a locked git dependency is downloaded correctly.

sdk/lib/_internal/pub/lib/src/git.dart

Index: sdk/lib/_internal/pub/lib/src/git.dart
diff --git a/sdk/lib/_internal/pub/lib/src/git.dart b/sdk/lib/_internal/pub/lib/src/git.dart
index e94e493e82042fc111ce3d6a40604f8667d724f0..4e92a698b6a6a7ecdeb2dbe74a21cd05483ec398 100644
--- a/sdk/lib/_internal/pub/lib/src/git.dart
+++ b/sdk/lib/_internal/pub/lib/src/git.dart
@@ -6,9 +6,13 @@
 library pub.git;
 
 import 'dart:async';
+import 'dart:io';
+
 import 'io.dart';
 import 'log.dart' as log;
 
+import 'package:stack_trace/stack_trace.dart';
+
 /// Tests whether or not the git command-line app is available for use.
 Future<bool> get isInstalled {
   if (_isGitInstalledCache != null) {
@@ -67,8 +71,13 @@ Future<bool> _tryGitCommand(String command) {
   return runProcess(command, ["--version"]).then((results) {
     var regexp = new RegExp("^git version");
     return results.stdout.length == 1 && regexp.hasMatch(results.stdout[0]);
-  }).catchError((err) {
+  }).catchError((err, stackTrace) {
     // If the process failed, they probably don't have it.
-    return false;
+    if (err is ProcessException) {
+      log.io('Git command is not "$command": $err\n$stackTrace');
+      return false;
+    }
+
+    throw err;
   });
 }