fix(@schematics/angular): migrate build-angular for 7.0 RC users

Fixes #12647

Author: clydin

packages/schematics/angular/migrations/migration-collection.json

@@ -14,6 +14,11 @@
       "version": "7.0.0-rc.0",
       "factory": "./update-7/index#polyfillMetadataRule",
       "description": "Update an Angular CLI project to version 7."
+    },
+    "migration-04": {
+      "version": "7.0.3",
+      "factory": "./update-7",
+      "description": "Update an Angular CLI project to version 7."
     }
   }
 }

packages/schematics/angular/migrations/update-7/index.ts

@@ -19,8 +19,11 @@ export { polyfillMetadataRule } from './polyfill-metadata';
 export default function(): Rule {
   return (tree, context) => {
     const existing = getPackageJsonDependency(tree, '@angular-devkit/build-angular');
-    const type = existing ? existing.type : NodeDependencyType.Dev;
+    if (existing && latestVersions.DevkitBuildAngular === existing.version) {
+      return;
+    }
 
+    const type = existing ? existing.type : NodeDependencyType.Dev;
     addPackageJsonDependency(
       tree,
       {