fix(stepfunctions): Choice state does not allow state input as a cond…

…ition (aws#8991)

Choice state Conditions are currently restricted to variables that must start
with $. or $[, however, the state input of $ is also valid.

Conditions evaluate against strings. Currently, we only allow attributes in the 
state input, but not the entire state input. The state input can be an object or
a plain string (i.e. "No messages").

This change relaxes the validation on the Condition to allow the state input
'$' to be supplied. This allows a string in a state input to be used in a condition
when using Choice states


Closes aws#8990

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-stepfunctions/lib/condition.ts

@@ -177,8 +177,8 @@ enum CompoundOperator {
 class VariableComparison extends Condition {
   constructor(private readonly variable: string, private readonly comparisonOperator: ComparisonOperator, private readonly value: any) {
     super();
-    if (!/^\$[.[]/.test(variable)) {
-      throw new Error(`Variable reference must start with '$.' or '$[', got '${variable}'`);
+    if (!/^\$|(\$[.[])/.test(variable)) {
+      throw new Error(`Variable reference must be '$', start with '$.', or start with '$[', got '${variable}'`);
     }
   }
 

packages/@aws-cdk/aws-stepfunctions/test/condition.test.ts

@@ -11,6 +11,9 @@ describe('Condition Variables', () => {
   test('Condition variables can start with $[', () => {
     expect(() => stepfunctions.Condition.stringEquals('$[0]', 'a')).not.toThrow();
   }),
+  test('Condition variables can reference the state input $', () => {
+    expect(() => stepfunctions.Condition.stringEquals('$', 'a')).not.toThrow();
+  }),
   test('NotConditon must render properly', () => {
     assertRendersTo(stepfunctions.Condition.not(stepfunctions.Condition.stringEquals('$.a', 'b')), { Not: { Variable: '$.a', StringEquals: 'b' } });
   }),