The current Dart SDK version is 2.10.2.

Because every version of flutter_test from sdk depends on test_api 0.2.19-nullsafety.2 and test >=1.15.3 <1.16.0-nullsafety depends on test_api 0.2.18, flutter_test from sdk is incompatible with test >=1.15.3 <1.16.0-nullsafety.
Because test >=1.16.0-nullsafety <1.16.0-nullsafety.5 requires SDK version >=2.10.0-0 <2.10.0 and test >=1.16.0-nullsafety.5 depends on pedantic >=1.10.0-nullsafety <1.10.0, test >=1.16.0-nullsafety requires pedantic >=1.10.0-nullsafety <1.10.0.
Thus, if flutter_test any from sdk and test >=1.15.3 then pedantic >=1.10.0-nullsafety <1.10.0.
And because syncfusion_flutter_core >=18.2.54 depends on pedantic >=1.9.0 <1.9.9, one of flutter_test any from sdk or test >=1.15.3 or syncfusion_flutter_core >=18.2.54 must be false.
And because stack_app depends on syncfusion_flutter_charts ^18.3.44 which depends on syncfusion_flutter_core ^18.3.44, flutter_test from sdk is incompatible with test >=1.15.3.
So, because stack_app depends on both flutter_test any from sdk and test ^1.15.4, version solving failed.
pub get failed (1; So, because stack_app depends on both flutter_test any from sdk and test ^1.15.4, version solving failed.)
Process finished with exit code 1

$ flutter doctor -v
[√] Flutter (Channel stable, 1.22.2, on Microsoft Windows [Version 10.0.19041.572], locale en-US)
    • Flutter version 1.22.2 at C:\Users\yash\flutter
    • Framework revision 84f3d28555 (2 weeks ago), 2020-10-15 16:26:19 -0700
    • Engine revision b8752bbfff
    • Dart version 2.10.2


[√] Android toolchain - develop for Android devices (Android SDK version 30.0.2)
    • Android SDK at C:\Users\yash\AppData\Local\Android\Sdk
    • Platform android-30, build-tools 30.0.2
    • ANDROID_HOME = C:\Users\yash\AppData\Local\Android\Sdk
    • Java binary at: C:\Program Files\Android\Android Studio\jre\bin\java
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b01)
    • All Android licenses accepted.

[√] Android Studio (version 4.0)
    • Android Studio at C:\Program Files\Android\Android Studio
    • Flutter plugin installed
    • Dart plugin version 193.7547
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b01)

[√] VS Code (version 1.50.1)
    • VS Code at C:\Users\yash\AppData\Local\Programs\Microsoft VS Code
    • Flutter extension version 3.15.1