fix: Interval evaluation of exp requires exponent to be a natural number

Author: fkleon

lib/src/evaluator.dart

@@ -549,6 +549,7 @@ class IntervalEvaluator extends ExpressionEvaluator<Interval> {
     var (exp, base) = pop2();
 
     // Expect exponent to be a natural number.
+    assert(exp.min == exp.max);
     int exponent = exp.min.toInt();
     num evalMin, evalMax;
 

test/evaluator_interval_test_set.dart

@@ -116,7 +116,7 @@ class IntervalEvaluatorTests extends TestSet {
       i1 * i2: int1 * int2,
       i1 / i2: int1 / int2,
       i1 % i2: throwsA(isUnimplementedError),
-      i1 ^ i2: Interval(57.6650390625, 3125.0),
+      i1 ^ n2: Interval(57.6650390625, 3125.0),
     };
     parameterized(cases);
   }