refactor(@angular/build): allow development server buildTarget option 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`.

Author: clydin

goldens/public-api/angular/build/index.api.md

@@ -110,7 +110,7 @@ export enum BuildOutputFileType {
 
 // @public
 export interface DevServerBuilderOptions {
-    buildTarget: string;
+    buildTarget?: string;
     headers?: {
         [key: string]: string;
     };

packages/angular/build/src/builders/dev-server/options.ts

@@ -36,8 +36,10 @@ export async function normalizeOptions(
 
   const cacheOptions = normalizeCacheOptions(projectMetadata, workspaceRoot);
 
-  // Target specifier defaults to the current project's build target using a development configuration
-  const buildTargetSpecifier = options.buildTarget ?? `::development`;
+  // Target specifier defaults to the current project's build target using the provided dev-server
+  // configuration if a configuration is present or the 'development' configuration if not.
+  const buildTargetSpecifier =
+    options.buildTarget ?? `::${context.target?.configuration || 'development'}`;
   const buildTarget = targetFromTargetString(buildTargetSpecifier, projectName, 'build');
 
   // Get the application builder options.

packages/angular/build/src/builders/dev-server/schema.json

@@ -111,5 +111,5 @@
     }
   },
   "additionalProperties": false,
-  "required": ["buildTarget"]
+  "required": []
 }