Add conditional operator (#172)

Author: natebosch

CHANGELOG.md

@@ -85,6 +85,7 @@ void main() {
 ```
 
 * Added `nullSafeProperty` to `Expression` to access properties with `?.`
+* Added `conditional` to `Expression` to use the ternary operator `? : `
 
 ## 2.2.0
 

lib/src/specs/expression.dart

@@ -76,6 +76,14 @@ abstract class Expression implements Spec {
     );
   }
 
+  Expression conditional(Expression whenTrue, Expression whenFalse) {
+    return new BinaryExpression._(
+      expression,
+      new BinaryExpression._(whenTrue, whenFalse, ':'),
+      '?',
+    );
+  }
+
   /// This expression preceded by `await`.
   Expression get awaited {
     return new BinaryExpression._(

test/specs/code/expression_test.dart

@@ -358,4 +358,11 @@ void main() {
       equalsDart('foo is! String'),
     );
   });
+
+  test('should emit a conditional', () {
+    expect(
+      refer('foo').conditional(literal(1), literal(2)),
+      equalsDart('foo ? 1 : 2'),
+    );
+  });
 }