build: modularize the gulp file to be easier to maintain (#14259)

This is a precursor to bringing in some of the gulp tasks from angular.io,
which could lead to the gulpfile becoming unwieldy.

tools/gulp-tasks/tools-build.js

@@ -0,0 +1,15 @@
+function tsc(projectPath, done) {
+  const path = require('path');
+  const platformScriptPath = require('./platform-script-path');
+  const childProcess = require('child_process');
+
+  childProcess
+      .spawn(
+          path.join(__dirname, platformScriptPath('../../node_modules/.bin/tsc')),
+          ['-p', path.join(__dirname, '../..', projectPath)], {stdio: 'inherit'})
+      .on('close', done);
+}
+
+module.exports = (gulp) => (done) => {
+  tsc('tools/', done);
+};