feat(@angular-devkit/build-angular): allow optional karma watch

Fixes angular#847

Author: clydin

packages/angular_devkit/build_angular/src/angular-cli-files/plugins/karma.ts

@@ -113,8 +113,8 @@ const init: any = (config: any, emitter: any, customFileHandlers: any) => {
   delete webpackConfig.entry.styles;
 
   // The webpack tier owns the watch behavior so we want to force it in the config.
-  webpackConfig.watch = options.watch;
-  if (!options.watch) {
+  webpackConfig.watch = !config.singleRun;
+  if (config.singleRun) {
     // There's no option to turn off file watching in webpack-dev-server, but
     // we can override the file watcher instead.
     webpackConfig.plugins.unshift({

packages/angular_devkit/build_angular/src/karma/index.ts

@@ -59,9 +59,11 @@ export class KarmaBuilder implements Builder<KarmaBuilderSchema> {
         // TODO: adjust options to account for not passing them blindly to karma.
         // const karmaOptions: any = Object.assign({}, options);
         // tslint:disable-next-line:no-any
-        const karmaOptions: any = {
-          singleRun: !options.watch,
-        };
+        const karmaOptions: any = {};
+
+        if (options.watch !== undefined) {
+          karmaOptions.singleRun = !options.watch;
+        }
 
         // Convert browsers from a string to an array
         if (options.browsers) {

packages/angular_devkit/build_angular/src/karma/schema.json

@@ -74,8 +74,7 @@
     },
     "watch": {
       "type": "boolean",
-      "description": "Run build when files change.",
-      "default": false
+      "description": "Run build when files change."
     },
     "poll": {
       "type": "number",