chore: fix modules/build.dart/pubspec

This was failing on Travis because the `pub install` would run before
copying of `pubspec.yml` happened. In fact, I don’t understand how this
worked at all because `gulp.dest` seems to be not forwarding files and
so anything after `gulp.dest` does not get called at all.

Here is the failing Travis build:
https://travis-ci.org/angular/angular/builds/40005692

This changes `modules/build.dart/pubspec` task to use `gulp-changed`
instead of a custom implementation and use a wrapper around `gulp.dest`
to forward files.

gulpfile.js

@@ -18,6 +18,7 @@ var shell = require('gulp-shell');
 var spawn = require('child_process').spawn;
 var through2 = require('through2');
 var watch = require('gulp-watch');
+var changed = require('gulp-changed');
 
 var js2es5Options = {
   sourceMaps: true,
@@ -44,6 +45,8 @@ var js2dartOptions = {
   outputLanguage: 'dart'
 };
 
+var PUB_CMD = process.platform === 'win32' ? 'pub.bat' : 'pub';
+
 var gulpTraceur = require('./tools/transpiler/gulp-traceur');
 
 // ---------
@@ -92,29 +95,14 @@ gulp.task('modules/build.dart/src', function() {
 gulp.task('modules/build.dart/pubspec', function(done) {
   var outputDir = sourceTypeConfigs.dart.outputDir;
   return gulp.src('modules/*/pubspec.yaml')
+    .pipe(changed(outputDir)) // Only forward files that changed.
+    .pipe(gulpDestWithForward(outputDir))
     .pipe(through2.obj(function(file, enc, done) {
-      var targetFile = path.join(outputDir, file.relative);
-      if (fs.existsSync(targetFile)) {
-        file.previousContents = fs.readFileSync(targetFile);
-      } else {
-        file.previousContents = '';
-      }
-      this.push(file);
-      done();
-    }))
-    .pipe(gulp.dest(outputDir))
-    .pipe(through2.obj(function(file, enc, done) {
-      if (file.previousContents.toString() !== file.contents.toString()) {
-        console.log(file.path + ' changed, calling pub get');
-        var pubCmd = (process.platform === "win32" ? "pub.bat" : "pub");
-        var stream = spawn(pubCmd, ['get'], {
+      // After a `pubspec.yml` is copied, we run `pub install`.
+      spawn(PUB_CMD, ['get'], {
         stdio: [process.stdin, process.stdout, process.stderr],
         cwd: path.dirname(file.path)
-        });
-        stream.on('close', done);
-      } else {
-        done();
-      }
+      }).on('close', done);
     }));
 });
 
@@ -160,6 +148,23 @@ function createModuleTask(sourceTypeConfig) {
   return mergeStreams(transpile, copy, html);
 }
 
+
+// Act as `gulp.dest()` but does forward the files to the next stream.
+// I believe this is how `gulp.dest` should work.
+function gulpDestWithForward(destDir) {
+  var stream = gulp.dest(destDir);
+  var originalTransform = stream._transform;
+
+  stream._transform = function(file, enc, done) {
+    return originalTransform.call(this, file, enc, function() {
+      stream.push(file);
+      done();
+    });
+  };
+
+  return stream;
+}
+
 // ------------------
 // ANALYZE
 

package.json

@@ -12,6 +12,7 @@
     "systemjs": "^0.9.1",
     "angular-benchpress": "^0.1.3",
     "gulp": "^3.8.8",
+    "gulp-changed": "^1.0.0",
     "gulp-rename": "^1.2.0",
     "gulp-watch": "^1.0.3",
     "gulp-shell": "^0.2.10",