The `serve` command will now use a default project target and builder
name if the target entry is not explicitly defined. This allows the removal
of additional configuration from an `angular.json` file. If the target is
already present than it will take priority over any default builder behavior.
The default logic will use the appropriate development server builder for
officially supported packages (`@angular/build`/`@angular-devkit/build-angular`).
The `dev-server` builder from these packages will currently assume a `development`
configuration is present within the `build` target. The default behavior may
be expanded in the future to support more arbitrary `build` target configurations.
If there is third-party package usage within the `build` target, the CLI
will attempt to discover a `dev-server` builder within the same package used
by the `build` target. If none is found, no default will be added and the
`serve` command will issue an error when no explicit target is present.

…on to be optional

The development server (`dev-server`) within `@angular/build` no longer
requires a `buildTarget` option to be specified. If not present, the value
will default to the current project's 'build' target. The configuration used
for the build target will be the same as the development server configuration
if specified or, if not specified, it will default to `development`.

…ildTarget` option to be optional

The development server (`dev-server`) within `@angular-devkit/build-angular` no longer
requires a `buildTarget` option to be specified. If not present, the value
will default to the current project's 'build' target. The configuration used
for the build target will be the same as the development server configuration
if specified or, if not specified, it will default to `development`.