put containsEsc on state rather than as a local variable

packages/babylon/src/tokenizer/state.js

@@ -54,6 +54,12 @@ export default class State {
     this.context = [ct.b_stat];
     this.exprAllowed = true;
 
+    // Used to signal to callers of `readWord1` whether the word
+    // contained any escape sequences. This is needed because words with
+    // escape sequences must not be interpreted as keywords.
+
+    this.containsEsc = false;
+
     return this;
   }