feat(build): add tslint to the build.

The first enabled rule enforces return types
declared on non-private (underscore-prefix)
methods that return something.

gulpfile.js

@@ -37,6 +37,7 @@ var replace = require('gulp-replace');
 var insert = require('gulp-insert');
 var uglify = require('gulp-uglify');
 var shouldLog = require('./tools/build/logging');
+var tslint = require('gulp-tslint');
 
 require('./tools/check-environment')({
   requiredNpmVersion: '>=2.9.0',
@@ -246,6 +247,19 @@ gulp.task('enforce-format', function() {
   });
 });
 
+gulp.task('lint', ['build.tools'], function() {
+  // https://github.com/palantir/tslint#supported-rules
+  var tslintConfig = {
+    "rules": {
+      "requireReturnType": true
+    }
+  };
+
+  return gulp.src(['modules/angular2/src/**/*.ts', '!modules/angular2/src/test_lib/**'])
+      .pipe(tslint({configuration: tslintConfig, rulesDirectory: 'dist/tools/tslint'}))
+      .pipe(tslint.report('prose'));
+});
+
 // ------------
 // check circular dependencies in Node.js context
 gulp.task('build/checkCircularDependencies', function (done) {
@@ -592,7 +606,7 @@ gulp.task('pre-test-checks', function(done) {
 });
 
 gulp.task('post-test-checks', function(done) {
-  runSequence('enforce-format', sequenceComplete(done));
+  runSequence('lint', 'enforce-format', sequenceComplete(done));
 });