@def WIDTH 10px;
@def HEIGHT 10px;
@def COLOR white;

/* will be considered as outside the conditional */
/* Compiler fails when lenient mode is disable */
@if (eval("com.google.gwt.resources.client.gss.BooleanEval.FIRST")) {
  @def WIDTH 15px;
}

@if (is("customProperty", "true")) {
  @def COLOR black;
}

@if (!is("customProperty", "true")) {
  @def HEIGHT 15px;
}

.foo {
  width: WIDTH;
  height: HEIGHT;
  color: COLOR;
}