fix(cli): Disable line folding in YAML (#2964)

Certain versions of YAML support long line folding, however the
CloudFormation YAML parser does not handle those. Disabling folding
when generating YAML so that we keep generating correct templates.

Fixes #2703

packages/aws-cdk/lib/serialize.ts

@@ -1,10 +1,19 @@
 import YAML = require('yaml');
 
+// tslint:disable-next-line: no-var-requires
+const yamlTypes = require('yaml/types');
+
 /**
  * Stringify to YAML
  */
 export function toYAML(obj: any): string {
+  const oldFold = yamlTypes.strOptions.fold.lineWidth;
+  try {
+    yamlTypes.strOptions.fold.lineWidth = 0;
     return YAML.stringify(obj, { schema: 'yaml-1.1' });
+  } finally {
+    yamlTypes.strOptions.fold.lineWidth = oldFold;
+  }
 }
 
 /**
@@ -35,4 +44,4 @@ export function serializeStructure(object: any, json: boolean) {
   } else {
     return toYAML(object);
   }
-}
+}

packages/aws-cdk/test/test.serialize.ts

@@ -0,0 +1,12 @@
+import nodeunit = require('nodeunit');
+import { toYAML } from '../lib/serialize';
+
+export = nodeunit.testCase({
+  toYAML: {
+    'does not wrap lines'(test: nodeunit.Test) {
+      const longString = 'Long string is long!'.repeat(1_024);
+      test.equal(toYAML({ longString }), `longString: ${longString}\n`);
+      test.done();
+    }
+  }
+});