Run `pub get` if flutter_tools.stamp changes (#3206)

This will help folks run pub get after they upgrade their flutter.git
revision.

packages/flutter_tools/lib/src/dart/pub.dart

diff --git a/packages/flutter_tools/lib/src/dart/pub.dart b/packages/flutter_tools/lib/src/dart/pub.dart
index 49a173d..9091614 100644
--- a/packages/flutter_tools/lib/src/dart/pub.dart
+++ b/packages/flutter_tools/lib/src/dart/pub.dart
@@ -9,8 +9,21 @@
 
 import '../base/logger.dart';
 import '../base/process.dart';
+import '../cache.dart';
 import '../globals.dart';
 
+bool _shouldRunPubGet({ File pubSpecYaml, File dotPackages }) {
+  if (!dotPackages.existsSync())
+    return true;
+  DateTime dotPackagesLastModified = dotPackages.lastModifiedSync();
+  if (pubSpecYaml.lastModifiedSync().isAfter(dotPackagesLastModified))
+    return true;
+  File flutterToolsStamp = new File(path.join(Cache.instance.getRoot().path, 'flutter_tools.stamp'));
+  if (flutterToolsStamp.lastModifiedSync().isAfter(dotPackagesLastModified))
+    return true;
+  return false;
+}
+
 Future<int> pubGet({
   String directory,
   bool skipIfAbsent: false,
@@ -30,7 +43,7 @@
     return 1;
   }
 
-  if (!checkLastModified || !dotPackages.existsSync() || pubSpecYaml.lastModifiedSync().isAfter(dotPackages.lastModifiedSync())) {
+  if (!checkLastModified || _shouldRunPubGet(pubSpecYaml: pubSpecYaml, dotPackages: dotPackages)) {
     String command = upgrade ? 'upgrade' : 'get';
     Status status = logger.startProgress("Running 'pub $command' in ${path.basename(directory)}...");
     int code = await runCommandAndStreamOutput(