Fix double.infinity serialization (#40099)

packages/flutter/lib/src/foundation/diagnostics.dart

@@ -2669,7 +2669,11 @@ class DiagnosticsProperty<T> extends DiagnosticsNode {
     json['defaultLevel'] = describeEnum(_defaultLevel);
     if (value is Diagnosticable || value is DiagnosticsNode)
       json['isDiagnosticableValue'] = true;
-    if (value is num || value is String || value is bool || value == null)
+    if (v is num)
+      // Workaround for https://github.com/flutter/flutter/issues/39937#issuecomment-529558033.
+      // JSON.stringify replaces infinity and NaN with null.
+      json['value'] = v.isFinite ? v :  v.toString();
+    if (value is String || value is bool || value == null)
       json['value'] = value;
     return json;
   }

packages/flutter/test/foundation/diagnostics_test.dart

@@ -1050,6 +1050,15 @@ void main() {
     validateDoublePropertyJsonSerialization(doubleWithUnit);
   });
 
+  test('double.infinity serialization test', () {
+    final DoubleProperty infProperty1 = DoubleProperty('double1', double.infinity);
+    validateDoublePropertyJsonSerialization(infProperty1);
+    expect(infProperty1.toString(), equals('double1: Infinity'));
+
+    final DoubleProperty infProperty2 = DoubleProperty('double2', double.negativeInfinity);
+    validateDoublePropertyJsonSerialization(infProperty2);
+    expect(infProperty2.toString(), equals('double2: -Infinity'));
+  });
 
   test('unsafe double property test', () {
     final DoubleProperty safe = DoubleProperty.lazy(