Skip parsing if file doesn't contain assertions

Unassertify is quite an expensive transform, because it does full
parsing and code generation. That only needs to be done for files that
contain assertions, though. This patch adds a quick check: if the string
`assert` does not even occur in the file, we'll just skip it, saving
some time.

index.js

@@ -87,6 +87,11 @@ function shouldProduceSourceMap (options) {
     return (options && options._flags && options._flags.debug);
 }
 
+function containsAssertions (src) {
+    // Matches both `assert` and `power-assert`.
+    return src.indexOf('assert') !== -1;
+}
+
 module.exports = function unassertify (filepath, options) {
     if (path.extname(filepath) === '.json') {
         return through();
@@ -100,11 +105,13 @@ module.exports = function unassertify (filepath, options) {
     }
 
     function end() {
-        if (shouldProduceSourceMap(options)) {
+        if (!containsAssertions(data)) {
+            stream.queue(data);
+        } else if (shouldProduceSourceMap(options)) {
             stream.queue(applyUnassertWithSourceMap(data, filepath));
         } else {
             stream.queue(applyUnassertWithoutSourceMap(data));
-        }        
+        }
         stream.queue(null);
     }
 

test/fixtures/func/no-assert.js

@@ -0,0 +1 @@
+contains some invalid code to make sure that this did not get parsed

test/test.js

@@ -43,6 +43,17 @@ describe('unassertify', function () {
             done();
         }));
     });
+    it('skips files that do not contain assertions', function (done) {
+        var filename = path.join(__dirname, 'fixtures', 'func', 'no-assert.js');
+        fs.createReadStream(filename)
+            .pipe(unassertify(filename, {}))
+            .pipe(es.wait(function(err, data) {
+                assert(!err);
+                var code = data.toString('utf-8');
+                assert(! /assert/.test(code));
+                done();
+            }));
+    });
 });