[Sass] Add a method to assert that a SassScript value is an integer.

lib/sass/script/literal.rb

@@ -170,5 +170,8 @@ module Sass::Script
     def to_i
       raise Sass::SyntaxError.new("#{self.inspect} is not an integer.")
     end
+
+    # @raise [Sass::SyntaxError] if this literal isn't an integer
+    def assert_int!; to_i; end
   end
 end

lib/sass/tree/for_node.rb

@@ -30,9 +30,10 @@ module Sass::Tree
     def _perform(environment)
       from = @from.perform(environment)
       to = @to.perform(environment)
-      if to.respond_to?(:coerce)
-        to = to.send(:coerce, from.numerator_units, from.denominator_units)
-      end
+      from.assert_int!
+      to.assert_int!
+
+      to = to.send(:coerce, from.numerator_units, from.denominator_units)
       range = Range.new(from.to_i, to.to_i, @exclusive)
 
       children = []

test/sass/engine_test.rb

@@ -76,6 +76,7 @@ class SassEngineTest < Test::Unit::TestCase
     "@if false\n@else if " => "Invalid else directive '@else if': expected 'if <expr>'.",
     "a\n  !b = 12\nc\n  d = !b" => 'Undefined variable: "!b".',
     "=foo\n  !b = 12\nc\n  +foo\n  d = !b" => 'Undefined variable: "!b".',
+    '@for !a from "foo" to 1' => '"foo" is not an integer.',
     '@for !a from 1 to "2"' => '"2" is not an integer.',
     '@for !a from 1 to "foo"' => '"foo" is not an integer.',
     '@for !a from 1 to 1.232323' => '1.232 is not an integer.',