refactor(build): replace source in environments with environmentDefault entry

Replace source in environments with separate environmentDefault entry and updated docs to reflect this change

BREAKING CHANGE: angular-cli.json has changed. A new environmentDefault entry replaces the previous source entry inside environments.

To migrate the code follow the example below:

Before:

"environments": {
"source": "environments/environment.ts",
	"dev": "environments/environment.ts",
	"prod": "environments/environment.prod.ts"
}

After:

"environmentSource": "environments/environment.ts",
"environments": {
	"dev": "environments/environment.ts",
	"prod": "environments/environment.prod.ts"
}

Closes angular#3857

Author: jsanchezgarcia

packages/angular-cli/blueprints/ng2/files/angular-cli.json

@@ -21,8 +21,8 @@
         "styles.<%= styleExt %>"
       ],
       "scripts": [],
+      "environmentSource": "environments/environment.ts",
       "environments": {
-        "source": "environments/environment.ts",
         "dev": "environments/environment.ts",
         "prod": "environments/environment.prod.ts"
       }

packages/angular-cli/models/webpack-build-test.js

@@ -128,7 +128,7 @@ const getWebpackTestConfig = function (projectRoot, environment, appConfig, test
         // This plugin is responsible for swapping the environment files.
         // Since it takes a RegExp as first parameter, we need to escape the path.
         // See https://webpack.github.io/docs/list-of-plugins.html#normalmodulereplacementplugin
-        new RegExp(path.resolve(appRoot, appConfig.environments['source'])
+        new RegExp(path.resolve(appRoot, appConfig['environmentSource'])
           .replace(/[\-\[\]\/\{\}\(\)\*\+\?\.\\\^\$\|]/g, '\\$&')),
         path.resolve(appRoot, appConfig.environments[environment])
       ),