Fix bug with evaluating an expression in it's own binding

packages/babel-core/test/fixtures/plugins/multiple-definition-evaluation/exec.js

@@ -0,0 +1,18 @@
+var code = multiline([
+  "function foo() {",
+  "  var a = a ? a : a;",
+  "}",
+]);
+
+transform(code, {
+  plugins: function (b) {
+    var t = b.types;
+    return {
+      visitor: {
+        ConditionalExpression: function(path) {
+          path.get("test").evaluateTruthy();
+        }
+      }
+    }
+  }
+});