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

Fixes #12647

Author: clydin

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

@@ -17,7 +17,11 @@
     },
     "migration-04": {
       "version": "7.1.0-beta.0",
-      "factory": "./update-7/index#typeScriptHelpersRule",
+      "factory": "./update-7/index#typeScriptHelpersRule"
+    },
+    "migration-05": {
+      "version": "7.0.3",
+      "factory": "./update-7",
       "description": "Update an Angular CLI project to version 7."
     }
   }

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

@@ -20,8 +20,11 @@ export { typeScriptHelpersRule } from './typescript-helpers';
 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,
       {