fix reentrant locking in pub upgrade (#4556)

packages/flutter_tools/lib/src/cache.dart

@@ -55,7 +55,7 @@ class Cache {
         locked = true;
       } on FileSystemException {
         if (!printed) {
-          printStatus('Waiting to be able to obtain lock of Flutter cache directory...');
+          printStatus('Waiting to be able to obtain lock of Flutter binary artifacts directory...');
           printed = true;
         }
         await new Future/*<Null>*/.delayed(const Duration(milliseconds: 50));
@@ -65,9 +65,8 @@ class Cache {
 
   /// Releases the lock. This is not necessary unless the process is long-lived.
   static void releaseLockEarly() {
-    if (!_lockEnabled)
+    if (!_lockEnabled || _lock == null)
       return;
-    assert(_lock != null);
     _lock.closeSync();
     _lock = null;
   }

packages/flutter_tools/lib/src/commands/upgrade.dart

@@ -45,10 +45,13 @@ class UpgradeCommand extends FlutterCommand {
       return code;
 
     // Check for and download any engine and pkg/ updates.
+    // We run the 'flutter' shell script re-entrantly here
+    // so that it will download the updated Dart and so forth
+    // if necessary.
     printStatus('');
     printStatus('Upgrading engine...');
     code = await runCommandAndStreamOutput(<String>[
-      'bin/flutter', '--no-color', 'precache'
+      'bin/flutter', '--no-lock', '--no-color', 'precache'
     ], workingDirectory: Cache.flutterRoot);
 
     printStatus('');

packages/flutter_tools/lib/src/runner/flutter_command_runner.dart

@@ -48,6 +48,11 @@ class FlutterCommandRunner extends CommandRunner {
         negatable: true,
         hide: !verboseHelp,
         help: 'Whether to use terminal colors.');
+    argParser.addFlag('lock',
+        negatable: true,
+        hide: !verboseHelp,
+        help: 'Whether to lock the Flutter binary artifacts directory while running. (This prevents multiple simultaneous Flutter instances from colliding.)',
+        defaultsTo: true);
     argParser.addFlag('suppress-analytics',
         negatable: false,
         hide: !verboseHelp,
@@ -142,7 +147,8 @@ class FlutterCommandRunner extends CommandRunner {
     // enginePath's initialiser uses it).
     Cache.flutterRoot = path.normalize(path.absolute(globalResults['flutter-root']));
 
-    await Cache.lock();
+    if (globalResults['lock']);
+      await Cache.lock();
 
     if (globalResults['suppress-analytics'])
       flutterUsage.suppressAnalytics = true;