fix(cli): Disable line folding in YAML (aws#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 aws#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();
+    }
+  }
+});