Cache the WidgetCreatorTracker.

This fixes the problem when the flutter bots run multiple tests from the flutter tool e.g.,
  flutter test first_widget_test.dart second_widget_test.dart

The first test causes the set of libraries, particularly Flutter's framework library, to be passed
to the transformer. The transformer needs the 'Widget' class located in Flutter's framework
library. The transformer on the first test works.  Each subsequent test only the libraries not in
the prior test are passed, without the framework library, the transformer is unable to find the
'Widget' class and the widget transformer fails to run.

This fix allows the use of a nice speedup to hot-reloading Flutter applications when
--track-widget-creation is enabled and it allows us to enable --track-widget-creation by default,
for command line Flutter users.

This fixes issue #36640

[email protected],[email protected]

Change-Id: I9a9c9dc69995122d0f7a7a3e1dfaebf57abb4afb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/105661
Commit-Queue: Terry Lucas <[email protected]>
Reviewed-by: Aske Simon Christensen <[email protected]>
Reviewed-by: Jacob Richman <[email protected]>

Author: terrylucas

pkg/dev_compiler/lib/src/kernel/target.dart

@@ -20,6 +20,8 @@ class DevCompilerTarget extends Target {
 
   ClassHierarchy hierarchy;
 
+  WidgetCreatorTracker _widgetTracker;
+
   @override
   bool get legacyMode => false;
 
@@ -117,7 +119,10 @@ class DevCompilerTarget extends Target {
       DiagnosticReporter diagnosticReporter,
       {void logger(String msg)}) {
     if (flags.trackWidgetCreation) {
-      WidgetCreatorTracker().transform(component, libraries);
+      if (_widgetTracker == null) {
+        _widgetTracker = WidgetCreatorTracker();
+      }
+      _widgetTracker.transform(component, libraries);
     }
   }
 

pkg/vm/lib/target/flutter.dart

@@ -12,6 +12,8 @@ import 'package:vm/target/vm.dart' show VmTarget;
 class FlutterTarget extends VmTarget {
   FlutterTarget(TargetFlags flags) : super(flags);
 
+  WidgetCreatorTracker _widgetTracker;
+
   @override
   String get name => 'flutter';
 
@@ -53,7 +55,10 @@ class FlutterTarget extends VmTarget {
       DiagnosticReporter diagnosticReporter,
       {void logger(String msg)}) {
     if (flags.trackWidgetCreation) {
-      new WidgetCreatorTracker().transform(component, libraries);
+      if (_widgetTracker == null) {
+        _widgetTracker = WidgetCreatorTracker();
+      }
+      _widgetTracker.transform(component, libraries);
     }
   }
 }