isJsRuntime top-level property
True when running on the JS runtime (Flutter web / dart2js / dartdevc).
Detection is via identical(0, 0.0): on the Dart VM the int 0 and the
double 0.0 are distinct objects (different runtime types), but on JS
both are the same Number, so the identity check passes there. Computed
once at load and held in a final to avoid per-call overhead.
Implementation
final bool isJsRuntime = identical(0, 0.0);