chore(schematics): better NgModule search logic (#14449)

* Similarly to https://github.com/angular/material2/pull/12636#discussion_r209788320, I want to improve the logic for finding `NgModule` declarations within a source file by using BFS.

src/cdk/schematics/utils/ast/ng-module-imports.ts

@@ -21,19 +21,7 @@ export function hasNgModuleImport(tree: Tree, modulePath: string, className: str
 
   const parsedFile = ts.createSourceFile(modulePath, moduleFileContent.toString(),
       ts.ScriptTarget.Latest, true);
-  let ngModuleMetadata: ts.ObjectLiteralExpression | null = null;
-
-  const findModuleDecorator = (node: ts.Node) => {
-    if (ts.isDecorator(node) && ts.isCallExpression(node.expression) &&
-        isNgModuleCallExpression(node.expression)) {
-      ngModuleMetadata = node.expression.arguments[0] as ts.ObjectLiteralExpression;
-      return;
-    }
-
-    ts.forEachChild(node, findModuleDecorator);
-  };
-
-  ts.forEachChild(parsedFile, findModuleDecorator);
+  const ngModuleMetadata = findNgModuleMetadata(parsedFile);
 
   if (!ngModuleMetadata) {
     throw new SchematicsException(`Could not find NgModule declaration inside: "${modulePath}"`);
@@ -66,6 +54,29 @@ function resolveIdentifierOfExpression(expression: ts.Expression): ts.Identifier
   return null;
 }
 
+/**
+ * Finds a NgModule declaration within the specified TypeScript node and returns the
+ * corresponding metadata for it. This function searches breadth first because
+ * NgModule's are usually not nested within other expressions or declarations.
+ */
+function findNgModuleMetadata(rootNode: ts.Node): ts.ObjectLiteralExpression | null {
+  // Add immediate child nodes of the root node to the queue.
+  const nodeQueue: ts.Node[] = [...rootNode.getChildren()];
+
+  while (nodeQueue.length) {
+    const node = nodeQueue.shift()!;
+
+    if (ts.isDecorator(node) && ts.isCallExpression(node.expression) &&
+        isNgModuleCallExpression(node.expression)) {
+      return node.expression.arguments[0] as ts.ObjectLiteralExpression;
+    } else {
+      nodeQueue.push(...node.getChildren());
+    }
+  }
+
+  return null;
+}
+
 /** Whether the specified call expression is referring to a NgModule definition. */
 function isNgModuleCallExpression(callExpression: ts.CallExpression): boolean {
   if (!callExpression.arguments.length ||