[flutter_tools] add --null-assertions flag for debugging with null safety (#63416)

packages/flutter_tools/lib/src/device.dart

diff --git a/packages/flutter_tools/lib/src/device.dart b/packages/flutter_tools/lib/src/device.dart
index c691610..2a16f8e 100644
--- a/packages/flutter_tools/lib/src/device.dart
+++ b/packages/flutter_tools/lib/src/device.dart
@@ -790,6 +790,7 @@
     this.webEnableExpressionEvaluation = false,
     this.vmserviceOutFile,
     this.fastStart = false,
+    this.nullAssertions = false,
    }) : debuggingEnabled = true;
 
   DebuggingOptions.disabled(this.buildInfo, {
@@ -821,7 +822,8 @@
       deviceVmServicePort = null,
       vmserviceOutFile = null,
       fastStart = false,
-      webEnableExpressionEvaluation = false;
+      webEnableExpressionEvaluation = false,
+      nullAssertions = false;
 
   final bool debuggingEnabled;
 
@@ -868,6 +870,8 @@
   final String vmserviceOutFile;
   final bool fastStart;
 
+  final bool nullAssertions;
+
   bool get hasObservatoryPort => hostVmServicePort != null;
 }
 
@@ -993,3 +997,14 @@
   @override
   Future<void> dispose() async { }
 }
+
+/// Append --null_assertions to any existing Dart VM flags if
+/// [debuggingOptions.nullAssertions] is true.
+String computeDartVmFlags(DebuggingOptions debuggingOptions) {
+  return <String>[
+    if (debuggingOptions.dartFlags?.isNotEmpty ?? false)
+      debuggingOptions.dartFlags,
+    if (debuggingOptions.nullAssertions)
+      '--null_assertions',
+  ].join(',');
+}