Simplify token context (#13450)

* refactor: unify tc.brace and tc.templateQuasi

* refactor: remove unused context check

* perf: reduce arrayPrototype call and hoist variables

packages/babel-parser/src/tokenizer/context.js

@@ -1,8 +1,7 @@
 // @flow
 
-// The token context is used to track whether `}` matches
-// a template quasi `${` or other tokens containing `{`:
-// namely tt.braceL `{` and tt.braceHashL `#{`
+// The token context is used to track whether the apostrophe "`"
+// starts or ends a string template
 
 import { types as tt } from "./types";
 
@@ -20,7 +19,6 @@ export const types: {
   [key: string]: TokContext,
 } = {
   brace: new TokContext("{"),
-  templateQuasi: new TokContext("${"),
   template: new TokContext("`", true),
 };
 
@@ -35,19 +33,22 @@ export const types: {
 // `this.prodParam` still has `[Yield]` production because it is not yet updated
 
 tt.braceR.updateContext = context => {
-  if (context.length > 1) {
-    context.pop();
-  }
+  context.pop();
 };
 
 // we don't need to update context for tt.braceBarL because we do not pop context for tt.braceBarR
-tt.braceL.updateContext = tt.braceHashL.updateContext = context => {
-  context.push(types.brace);
-};
-
-tt.dollarBraceL.updateContext = context => {
-  context.push(types.templateQuasi);
-};
+// ideally only dollarBraceL "${" needs a non-template context
+// in order to indicate that the last "`" in `${`" starts a new string template
+// inside a template element within outer string template.
+// but when we popped such context in `}`, we lost track of whether this
+// `}` matches a `${` or other tokens matching `}`, so we have to push
+// such context in every token that `}` will match.
+tt.braceL.updateContext =
+  tt.braceHashL.updateContext =
+  tt.dollarBraceL.updateContext =
+    context => {
+      context.push(types.brace);
+    };
 
 tt.backQuote.updateContext = context => {
   if (context[context.length - 1] === types.template) {

packages/babel-parser/src/tokenizer/state.js

@@ -127,10 +127,10 @@ export default class State {
   lastTokStart: number = 0;
   lastTokEnd: number = 0;
 
-  // The context stack is used to superficially track syntactic
-  // context to predict whether a regular expression is allowed in a
-  // given position.
+  // The context stack is used to track whether the apostrophe "`" starts
+  // or ends a string template
   context: Array<TokContext> = [ct.brace];
+  // Used to track whether a JSX element is allowed to form
   exprAllowed: boolean = true;
 
   // Used to signal to callers of `readWord1` whether the word