Significantly speed up gulp build --css-only (ampproject#9726)

Prior to this PR, gulp build --css-only would take ~ 1 minute to run, and end up building a bunch of js files in addition to compiling css.

After this PR, gulp build --css-only takes ~ 0.5 seconds to run (a 100x speed up), and the only output files in build/ after running it are .css, and .css.js files.

Resulting speeds on my workstation:
gulp build --css-only: ~500ms
gulp build: ~1m 45s
gulp dist --fortesting: ~3m 30s

Fixes ampproject#9640

build-system/tasks/compile.js

@@ -98,7 +98,9 @@ function compile(entryModuleFilenames, outputDir,
     var intermediateFilename = 'build/cc/' +
         entryModuleFilename.replace(/\//g, '_').replace(/^\./, '');
     if (!process.env.TRAVIS) {
-      util.log('Starting closure compiler for', entryModuleFilenames);
+      util.log(
+          'Starting closure compiler for',
+          util.colors.cyan(entryModuleFilenames));
     }
     // If undefined/null or false then we're ok executing the deletions
     // and mkdir.
@@ -340,8 +342,13 @@ function compile(entryModuleFilenames, outputDir,
         .pipe(gulp.dest(outputDir))
         .on('end', function() {
           if (!process.env.TRAVIS) {
-            util.log('Compiled', entryModuleFilename, 'to',
-                outputDir + '/' + outputFilename, 'via', intermediateFilename);
+            util.log(
+                'Compiled',
+                util.colors.cyan(entryModuleFilename),
+                'to',
+                outputDir + '/' + outputFilename,
+                'via',
+                intermediateFilename);
           }
           gulp.src(intermediateFilename + '.map')
               .pipe(rename(outputFilename + '.map'))