chore(build): use cheap-module-source-map for dev (#2619)

Allows for faster builds, partially addresses #1980

Author: filipesilva

Diff for: packages/angular-cli/models/webpack-build-development.ts

@@ -14,7 +14,7 @@ declare module 'webpack' {
 
 export const getWebpackDevConfigPartial = function(projectRoot: string, appConfig: any) {
   return {
-    devtool: 'source-map',
+    devtool: 'cheap-module-source-map',
     output: {
       path: path.resolve(projectRoot, appConfig.outDir),
       filename: '[name].bundle.js',